He will just test with a probe what direction the electricity is flowing and put positive to positive and negative to negative(Hopefully).
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you are right they don't have positive or negative sides, so why would you need to check them for polarity? but on a board and anywhere else electricity in a dc system only flows in one direction their for they would only need to find out what end of the current is positive and what is negative because electricity flows from negative to positive, and if they have something like a logic probe it will tell them when the current is high or low.