Ehhh...I think some online tests might give a good idea as to what issues you struggle with. Some tests are better than others. For example, I find Psychcentral's Sanity Score test is one of the better ones.

That being said, they are not meant to be a diagnostic tool! People who try a whole bunch of psych tests online and then self-diagnosing with a whole bunch of different things aren't helping their situation. If people are truly concerned about mental health issues, I personally think that seeing a professional in person is the best thing to do.