My doctor used a questionnaire in diagnosing my ADHD.
Adult ADHD Self-Report Scale
Before going to my doctor I had tried TotallyADD's virtual doctor.
Note that Dr. Russell Barkley says scoring high on hyperactivity (feeling driven like a motor) is of no diagnostic value in adults.
I have had difficulty finding objective online tests for ADHD. I've found studies showing people with ADHD perform below average on inhibition tests like go/no-go and stop signal. Studies indicate medication improves performance on those tests, and my tests on and off medication show an improvement too.
I also perform well below average (bottom 1%) on the n-back test, though I haven't found any studies of this. Although ADHD causes problems with working memory(WM), my initial score of 7 on the reverse digit span test indicate my WM capacity is above average. This paper concludes n-back doesn't primarily measure WM, rather it primarily measures attentional control. In other words the attention deficit makes it more difficult to effectively use WM. For me, medication doesn't seem to effect n-back performance.