Where the positive and negative side of an AC adapter?
A voltmeter will well you which lead is what. Otherwise, sometimes the cord will have a small 'ridge' on the + side, but that's not for certain.

An LED with a 1K or so load resistor in series can also work. But be warned that LEDs don't much like reverse polarity, you can protect it with a regular diode in parallel, wired in opposite polarity to the LED.

A lamp with a diode can work too.

Basically, a voltmeter would make your life easier.

Here's a way: prepare a container of salt water and get two small test tubes or a couple of small clear glasses. Fill the test tubes or glasses with the salt water, immerse them upside-down in the container. Run the two wires, one in each test tube. Turn on the power, watch the gases accumulate in the tubes. The one that fills quicker is H2, that's the (-) terminal.

To confirm, try lighting each tube with a match. The H2 tube should burn, while the other tube filled with O2 will not.

Here's the reaction that takes place (electrolysis):


Oxidation at anode: 2 H2O(l) → O2(g) + 4 H+(aq) + 4e.
Reduction at cathode: 4 H+(aq) + 4e− → 2 H2(g)


You'll see that for each O2(g), two 2 H2(g) are evolved, so there is twice as much H2 produced as O2, volume wise.