I get bloodwork done every month, even when I'm not on. This is the only way I can get real solid data on the effects these substances have on my physiology. Keeping track when I am off also gives me a good picture as to how well I recover.
I am a very lucky SOB though to be able to afford to do this. However I am very very happy I did; I have learned alot of things about the effects certain compounds have upon me that spits in the face of modern "truths" and is in some cases polar opposites of now accepted "facts".