I don't think I've ever seen a polarised capacitor, without a marking to determine the polarity. Please post a picture.
Electrolytic capacitors behave like poor diodes and will conduct, if reverse biased. Ensure it's fully discharged Connect it to 5V, via a 1k resistor. If it charges up normally, as expected, then the polarity is correct. If it sits at under a volt or so and never charges, then it will be reverse biased.