Free Carrier Lookup is a great site if you are using a paygo cell phone (or a small or regional carrier) to see which network you are using. Net10, StraightTalk, etc., now offer services on both Verizon and ATT. So to find out which carrier you are using, just enter your phone number and hit search.
Note: If you happen to get Cingular as the network, consider it ATT.
Link to site: http://freecarrierlookup.com/