You can get a referral to a psychiatrist from a doctor who will get you to fill a number of multi choice forms and look at your past school reports to form a diagnosis. This may be more or less involved depending on which country you live in, some places irresponsibly hand out adderall like candy.

IMHO the best approach if you suspect you may have ADHD is to do a lot of googling for adult ADHD, there are good videos on youtube, discussions on reddit and various forums. If you still think this might apply to you, then search for ADHD test.