Yes, tests exist but. No one "test" is determinative. An experienced mental health expert may or may not even utilize a proprietary "test". Adults can easily be evaluated. Such an evaluation includes a thorough history &discussion with the expert, who then can employ 1 or more tests, or not, to provide additional info. In the end, the evaluator should use tests with which he/she has confidence.
Biological disease . Being a biological disease, add has firm physical findings that can aid in its diagnosis. Neurobiological markers such as dyskinesia, temporal sequential ordering problems, eye dysmetria, synkinesia, poor working memory are easily discovered in a neurodevelopmental evaluation. There are "tests" for add! they are necessary for proper treatemnt and follow up.
Yes. Criteria for ADHD does include this in adults. Criteria comes from the DSM 5 which I can unfortunately not reproduce her given copy right. I would recommend googling "adult ADHD dsm5". No real "test" for ADHD and comes down to clinical/history based diagnosis.