How to test for ground loop using multimeter?
It's not a ground loop.  It's something else.  A loose connection, a broken connector or cable, or just crappy equipment.  Also, plug everything into the SAME power strip.  If that doesn't cure the problem then it absolutely isn't a ground loop.

"Ground Loops" almost never happen in the real world, and they absolutely don't happen with normal equipment plugged into the same wall outlet (via a power strip).