Easiest way is to put a volt meter on it.
Otherwise if you don't have anyone you can borrow from, just hook it up. With 5 volts dc, it isn't a safety issue. It will run backwards one way (reversed polarity).
Otherwise if you don't have anyone you can borrow from, just hook it up. With 5 volts dc, it isn't a safety issue. It will run backwards one way (reversed polarity).