How to check DC polarity using a multimeter
I just measured a Lithium battery with unknown polarity with a digital multimeter and got the following measurement. The same results should apply for other DC sources:



I hypothesized that the negative result may have meant that I'd attached the positive (red) lead of the multimeter to the negative lead of the battery. To confirm this hypothesis I flipped the battery leads around and found further evidence this may be true:



So it appears that if you connect a multimeter postive lead to positive, and negative to negative on the circuit under test the meter will give a positive measurement. If the connections are reversed you will get a negative voltage reading.