I have 2 wires running underground to a lamppost in my yard. I want to install a GFCI outlet in the post. To do so, I believe I have to determine which one is the neutral wire and which one is the hot wire. (Neither is colored) How can I determine which is which?

Just 2? No grounds ?

If you have a tester, voltage is best but just a small test light should work, run an extension cord from a known properly wired outlet and test from the unknown wires to the neutral on the extension cord (that's the longer slot). You should only get voltage on the hot wire.

As a side note I agree with mr500. Underground wire to an exterior lamp with no ground sounds like a bad thing. This lamp is 120volt right?

Just 2? No grounds ?

Yes, just 2 wires, there is no ground wire. It's an old house built in 1930. I've read that installing a GFCI outlet will make this lack of ground safer. That's why I'm trying to figure out which wire is the neutral wire. How do I do this?

Exactly what was stated.

CLARIFICATION OF THE HARD WAY

Find a ground somewhere. If you have to, use the outside of the meter base.

Another way is to find a properly grounded outlet if the house has been re-wired.

You can even use a cable connection or the ground on the NID for the phone service.

Run a wire such as an extension cord and measure the voltage between a known good ground and each of the wires. One will read 120 and the other 0. The two reading zero is neutral.

THE SIMPLE WAY

While writing this there is an EASY way to do this. Connect the wires to the GFCI. If the GFCI works, you have wired it correctly. If it doesn't you have wired it wrong. No damage will occur to the outlet if the polarity is wrong. Then, if you would like, you can use a simple, maybe $3.00 outlet tester to check your work.

The GFCI should come with a label that you can use when the outlet uses no ground. Use it.

I agree with Keepit!

How about since there is no ground, and a GFCI needs ground to measure against, you need to either pull a ground wire to the fixture or, use a GFCI breaker in place of the existing breaker.

Oops, is your electrical wiring protected by fuses of circuit breakers?

Don:

A GFCI DOES NOT NEED GROUND!. It measures the imbalance of hot and neutral. This is done by winding the hot conductor and neutral conduction in opposite directions of a torroidal core acting as a current transformer. Then, there is a sense winding which can very easily resolve the diference in currents.

In reality, reverse polarity, is picked up as a grounded neutral fault. Specifics can be found here:
http://www.national.com/ds/LM/LM1851.pdf#page=1

My error and I know better. I was thinking GFCI and wrote about GFI. My bad.