There’s not really a definitive way of knowing you have adhd without going to a doctor, honestly. I’d suggest doing a lot of research though, since that’s what helped me figure it out

looking up the DSM 5 criteria and asking people around you and yourself if you fit them

I’ve had adhd since I was 3 I have trouble learning things and staying focused on thing’s especially if I do not find it interesting I also have mood swings and it is really hard for me to learn things like math and science also I forgot to do important things or just don’t have the energy to do things like take a shower or or brush my teeth abs it’s hard for me to get things done especially on time

You can use the DSM-5 (it's easy to find a free pdf online) which is the manual for diagnosis. It's an easy way to see if you'd meet the criteria for being diagnosed and has genuinely helped multiple of my friends realize they have ADHD