Suppose you have a low voltage dc power supply whose terminals look
alike. How to find out which one is positive? Here is one suggestion,
made by chemist.
"A good test for the proper connection is to use a silver wire anode
and a platinum wire cathode in a sodium chloride solution. When
current is passed, you should see an insoluble white precipitate of
AgCl forming at the silver anode. If you don't see this, then your
cell is connected backwards."