What do doctors look for in blood tests?

What specifically do they look for? I guess part of the reason I am asking is do they check testosterone levels? I am getting blood tests done for another reason and think I may be testosterone deficient. This could be completely wrong but that is why I am wondering if they would notice in the tests. If not I will just consult my doctor on the issue, but it isn't something I would like to talk about if I don't have to.