If you knew how to use a multimeter you wouldnt ask these questions.
As you obviously dont know, you should first read up on how a multimeter works, and what are it's various settings.
The last thing you want to do is put it on CURRENT and then start touching power and grounds
Setting the multimeter to "continuity" the one that beeps at you... you can do that with the power off and there is no risk at all if you take antistatic protections.
However be careful in your diagnosis, working out what tracks are connected and what they go through will be difficult.
