Wiring not labeled (Red and Black)

Took off outdoor house lights - they were never updated - still old cloth covered wiring - but in good condition. After removing old fixtures, I realized, the wires were not labeled - they are both just black cloth over it.
I connected the wires anyway, based on recolleciton, and the lights worked fine.
However, is there a way to test if I have positive to positive, and negative to negative? I assume the lights would NOT work if I had the wiring backwards?
Thanks in advance for any insight.
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As long as there is not a photocell to turn them on and off, the lights will work fine either way. However, it is safer done properly. It is like the polarized plug on a lamp; it will work backwards, but you are more likely to get a shock off it when plugged in backwards. For $3 you can buy a voltage sensor that looks like a screwdriver. Touch the blade to the wire and your finger to the knob on the end; if the wire is hot, it will light up. It is dim, so you can't use it in full sunshine. (more expensive devices are much better, but I suspect you don't want to spend much?) With the switch on, but no bulb in the fixture, the hot (black) will light up, but the neutral (white) will not. Or, even simpler, with the bulb out, the threaded sleeve will not light up, and the knob at the back will. If they are the opposite, you have the fixtured wired backwards. The safety angle is that you are more likely to touch the sleeve than the knob. Okay?
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