The best way to check iron levels is to check ferritin, or iron storage. If your ferritin level is, in general, between 35-100 your iron levels are healthy, and you don’t need to supplement with it. Ideally, you would choose an iron-free multivitamin supplement because your levels are fine. If your ferritin level was below 35 you probably need more iron and you should choose a multivitamin that does contain iron. However, if your ferritin level is up past 100 and heading toward 150, you have way to much iron in your blood and it is probably causing oxidative damage. The standard (and healthy way) to lower your ferritin levels is to give blood, sometimes multiple times, until your ferritin levels go down again.
In the past decade, a study was done in which the ferritin levels of children with attention deficit disorder were tested, and they came back in the non-detected to 12 range. The children benefitted from supplementing with iron. Anemia is another commonly-known health condition caused by iron deficiency, but it can also be caused by low vitamin B12 or folic acid.