How can I determine which wire is positive/negative?

Question

I have a power supply and need to know which wire is positive and which is negative....

Is there a way to find this out without a meter (for which I don't have at the moment)?

Answer 1

If your power supply is DC, you can use an LED with a current limiting resistor.  Connect the LED/resistor to the power supply.  It does not light, reverse the + and - connections and it should light. Once the LED is illuminated, the cathode side of the LED is going to the negative terminal of your power supply