You are not in a position to know
To all the parents whose kids just told them that they're trans

Dear Parents,
Yes, you, the parent whose kid just came out to you. The parent whose kid just trusted you with something essential and utterly critical for their well-being. The parent who heard it, but doesn’t want to believe it.
Stop declaring that your kid isn’t trans.
I can hear the “Yes, but” forming on your lips. No! Listen for a minute. This is important.
I will grant you that it’s possible your kid isn't trans. They may well have a fair bit more introspecting to do, or work to do with a qualified therapist, in order to arrive at a final conclusion about their identity. So it’s possible that they're not trans. But it’s also possible that they are. It’s ok to be skeptical. But the point here is that you’re not the one who should or even can make that call.
This isn't kindergarten
Whether someone is male or female as a person, as a conscious being living in the world, isn’t directly connected to anatomy. Gender is more complex than the literal “explain like I’m five” version we give kids when they’re little: “boys have a penis and girls have a vagina.” Yes, it’s correlated with anatomy—most people really are cisgender—but anatomy is not the whole story.
There are two components to a person’s experience of gender. There’s “gender identity”, which is how you feel about yourself on the inside, in the deepest parts of your mind. And there’s “gender expression” (sometimes also called “gender presentation”), which is the combination of how your body is configured and whatever clothes and other styling you layer on top of that.
There is good evidence that both the internal and external aspects of a person’s gender gender are established during pregnancy. Gender identity is just as real as gender expression, and both arise from the natural, messy biology of fetal development. Both are just things you’re born with.
That bears repeating: your gender identity, that inner sense of self, is something you were born with. It’s just how your brain got wired as you were growing from an egg an infant.
But the kindergarten version doesn’t make any mention of gender identity, does it?
Most people never move beyond the kindergarten version of gender. To be fair, most people never need to. Most people really are cisgender; their outsides and their insides align. There’s no functional difference, as far as their identity goes, between what’s on the outside and how they feel about themselves on the inside. Many never even stop to consider that there might be a difference between those two things. And why would they? For cisgender folks, there’s very little need to do so.
Trans people, though, need to go beyond the kindergarten version of gender theory. We feel a clash between ourselves as people versus the way our bodies are configured and the entire way our lives have been built up around us on the basis of that configuration. We feel that clash, every moment of every day.
The duality of gender identity and gender expression is both critical and manifest to us, because the mis-alignment of the two explains this clash we feel, this subtle wrongness, about everything in our lives.
Feelings matter
Trans people become aware of our trans-ness, aware of this mismatch between our souls and our bodies, because that dissonance gives rise certain patterns of feelings. In short, these feelings are what we call gender dysphoria. I'm not going to explain all of gender dysphoria here. There are better resources for that.
The point is, we become aware that our inner self doesn’t match how the world sees us because of that feeling of subtle wrongness of living our lives in a way that isn’t true to who we actually are. Go read about gender dysphoria to learn more about how that actually feels.
The short answer is that it hurts. That subtle wrongness is soft, even tolerable, in any moment. But building up over all moments? It hurts. It builds and builds, gets worse and worse, until we literally feel like we are going to die or be driven insane if we don’t fix that mis-alignment between gender identity and gender expression.
That’s how your kid knows—or at least, suspects—that they're trans. Because of how they feel. Because it has built up to where they are now genuinely hurting. And their quest to understand why they are hurting has led them to realize that their body simply does not fit their soul.
Rejection
But too often, when kids share those feelings, parents reject them. Here is just one of many such stories I’ve seen, from the kids themselves:
I decided today that I would finally come out [as trans] to [my mom] and she literally just said “no you’re not”. After that she asked me why I thought I wasn’t a man. I explained to her that I never felt comfortable identifying as a man but then she told me that I was born in a male body with a penis which makes me a man. [Source]
It’s heartbreaking. This trans girl bares her soul to her mom, and what does the mom do? Rejects it. Responds with the simplistic kindergarten version of gender. As if that were the whole story. What happened next is predictable:
Im sitting in my room crying […], ranting to my friends over text about. She hasn’t talked to me since the conversation and honestly I don’t want to talk to her. I thought she might be confused or hesitant but she just outright ignored and disrespected how I feel. I dont know what to even think of her or myself anymore.
She’s is upset. She doesn’t trust her mom anymore. Doesn't even want to talk to her. This girl went to her mom for understanding, and what did she get? Rejection. Denial. Disrespect. Wouldn’t you be upset, too?
She wasn’t asking her mom to immediately take her to the doctor to get a prescription for hormones or anything like that. She just wanted to be heard. To be seen and loved and respected for who she knows herself to be. She just wanted a dialogue, and the mom shut her down.
I’m sure the mom was surprised. Shocked, even. But was that really the best way to respond? Was that a loving, compassionate response? The kid sure didn’t think so.
I hope the mom gets over it. I hope she’s able to do the work to repair the rift she just created between herself and her child. A child who is hurting, who knows why, and needs her mother’s help.
You are on the outside
The thing about feelings is that you only have access to your own. We are only human. We are not Star Trek’s Vulcans; we cannot mind-meld and directly share the subjective experience of each other’s lives. We are not telepathic. We are forever locked alone inside our own skulls.
As a parent, that can be hard. I have two kids. I’ve been there. I couldn’t even count the number of times it has been obvious that something was going on with them, but they didn’t want to talk about it, and I was left guessing about how they felt. That’s life. The best we can do to know our kid’s feelings is to earn their trust and then listen when they tell us.
Ultimately, you do not live inside your kid’s head. You do not know what it feels like to be them. Which means...
You are not in a position to know
Determining that a person is trans is about how they feel about themselves in their life. Being locked inside your own head, you just don’t have access to that information. You are quite simply not in a position to know. You are especially not in a position to know if you, yourself, are cisgender and have never experienced gender dysphoria for yourself.
But the other person is. Your kid is. They do live inside their own head. They get to feel their feelings directly. And if you’re lucky, if you’ve earned their trust, they might risk sharing those feelings with you in words.
When they do, how will you respond? I hope you won’t shut them down and declare that your kid isn’t trans. I hope you won't invalidate their feelings. I hope you won’t signal that you think you know them better than they know themselves. You don’t. You can’t. You’re on the outside.
So stop declaring. Start listening.
Your kid knows how they are feeling. They might not fully understand why. They might not be able to articulate it as well as they’d like. But they know how it feels to be them, living their life in their body.
If they choose to share that with you, that's a gift! What an incredible gift for a child to give their parent: the trust and openness to share their deepest feelings! Respect that. Honor that. Cherish that.
Listen to your kids when they tell you who they are. The best gift you can give them in return is to believe them. To have the humility to recognize that they are the expert when it comes to their own feelings. Not you. Figuring out what to do with that, how to best help them, can come later.
For now, just listen to what they are telling you.