Diagnosis
It’s fairly common for ADHD to be misdiagnosed. Often people are diagnosed with depression and/or anxiety when in reality they have ADHD. Untreated ADHD is a common cause of depression and anxiety.
ADHD is formally diagnosed by the DSM-5
You can do a assessment through ADHDAdulthood.com
It’s important to note that children and adults display symptoms differently. Women and girls are under-diagnosed. Girls/women can display symptoms differently than boys/men.
If you believe you or someone you know has it, you should talk with your doctor. Usually it is diagnosed by a psychiatrist. It’s recommended to see a psychiatrist because other disorders like bipolar disorder can be misdiagnosed as ADHD
Note: I’d also like to mention that each state within the United States, and different countries may have their own requirements for diagnosis. However, the main way to be diagnosed is with the DSM-5.