Huh? You don't have to be tested as-a-child, you don't have to call up your fourth grade teacher. You can self report your subjective experience.
"Did you have these problems as a child? Have you always had trouble doing your homework? Did stressing out about homework enable you to do the homework?"
They ask the question a few different ways typically.
I get it, perhaps it is different in your area, but I get the impression that an adult ADHD diagnosis is far from impossible.