This is sort of my concern. I had extremely low vitamin D, so I started megadosing on the advice of my doctor (50000IU/week). The problem is, without regular tests, I don't really know when I should be stopping. Vitamin D is fat soluble, and at some point, I'll in theory have levels that are too high.
True - you should just schedule a blood test. In most US states you can do it yourself without a physician via online lab ordering companies. You will pay out of pocket (or you can use health savings). I usually do this b/c its easier for stuff like this if I am curious about blood levels and procrastinate too much on setting up doctor appts ;)
EDIT: Wanted to mention that a Vit D test is usually around 30-50 bucks, so not too expensive