ADHD is hard to recognize.
It's not like a broken bone, you can't just do a simple X-ray and get a binary, scientifically confirmed diagnosis.
If you're seriously wondering if you have ADHD, the best thing to do is a see a doctor. Tell them your symptoms, and they will either send you to a specialist or treat you themselves.
That being said, here's some common "I didn't know other people did that" moments that I've found:
- Trouble focusing on most tasks
- Infrequent & random bursts of extreme, obsessive focus (we call this hyperfocus)
- Trying desperately to "force yourself" into hyperfocus every day, and becoming despondent when you just can't summon that on command.
- Purchasing equipment, and going all-in on some new hobby, and promptly dropping it soon after.
- Passionately starting a shiny new habit, and then totally forgetting about it the next day.
- Total inconsistency in all aspects of your life. School work, study habits, tasks at work, exercising, sleep schedule, you name it. Always inconsistent.
- Brain fog. Some days you wake up and you can't focus on anything. You try really hard, but you just can't get started on whatever your tasks are.
- Inability to do things you really want to do. You sit there starting at your computer monitor for hours, but you just can't get started on your work. You really want to do that work, but your brain just won't let you.
- Twitching/fidgeting. You constantly move around your chair, scratch your arm, spin a pen, move your mouse around your desktop randomly, bounce your leg up and down. All the time.
- Getting into arguments/debates. You hear something wrong or different and you just have to offer your opinion on the subject. You can hardly stop yourself, even though you regret it later.
- Irritability at seemingly innocent things. For example: some noise in the background that everyone else can seemingly ignore becomes a massive problem for you, which makes you furiously try to turn it off.
- A huge variety in abilities. You are the "jack of all trades" because you have tried dozens of different hobbies, and now have an intermediate level understanding in each of them. This would be an asset at work, if only you could find a way to stay focused on one task for a longer period of time.
- You think you're "lazy" or you have an "effort problem". This generally emerges after you observe your coworkers/peers/classmates being able to finish their work on time and start doing it whenever they want. Laziness is a choice - if you actively want to do work and be productive, it's probably not "laziness", but it's very easy to perceive it that way and blame yourself.
That's a pretty decent list, but it's not comprehensive. It's also not exhaustive, and not everything will apply. Also, you can have many of these without having ADHD.
These are just some general traits I've found after speaking to many ADHDers and making observations about them.
I read a book called ADHD Pro that details what it's like to have ADHD, its symptoms, and some great options on how to live your life in the most productive way while having ADHD.