Go into a tmobile store and ask a rep to see if they can put in their SIM to verify. Most reps at tmobile now have iPhones on site and the micro-sim is available as well as the nano-sim b/c tmobile now officially carriers both models (4s & 5).
Well the reason why I told him to try going to a tmobile store is so that some one there can do him the favor to put an activated tmobile sim to see if it picks up their network (to confirm it is unlocked), they have done it for me.
yea unless you can find someone else with another carriers SIM card, you can goto the store and ask a rep to check. also, try googling some of those IMEI checker sites. some of them also show the SIM lock status, but i dont know how reliable, if at all, they are.