Originally Posted by Bugaboos

If you can find a friend with
1. an unlocked GSM phone NOT on AT&T (T-mobile, etc); and
2. a sim adapter to make the micro-sim into a regular size sim - you can then test it.

Putting the sim into a unlocked GSM phone, it should find the T-mobile network.
There were probably some instructions on how to check your balance that came with the sim. This is usually a free call or text. By getting a signal, and getting a balance message, you can verify it's activated and how much credit you have. If you bought it activated, but with no credit (can you do that?), you'll have to assume if it finds the t-mobile network it's OK.

I just did this with a vodafone UK sim I purchased with 10 pounds credit from e-bay.