How to know if my Iphone is unlocked?

A while back I found an ATT iPhone 4 and recently decided to jailbreak it. I want to use my iPhone 4 on T-mobile, so how do I know if its unlocked?

If you have recently jailbroken this phone, odds are that it needs to be unlocked as well.

Beg/borrow/steal a T-Mobile SIM card that has already been cut down to the right size and insert it. If the phone is unlocked, the top left corner will read "T-Mobile" (active SIM card) or "No Service" (inactive SIM card). If you just get a blank with empty signal bars, then the phone is likely not yet unlocked.