How to figure out which carrier my Iphone is on?

I got an Iphone 4 not too long ago, bought it off of someone. I had to reboot and restore the phone and its back to the welcome setup page. I have my Bell sim card installed in but when I try to set up my phone, both on Itunes and on the phone itself quotes that my sim card is not with the proper carrier and to put in a card with the proper carrier. I was wondering if there would be a way to figure out which carrier it is on so I know which sim card is needed.

Why can't you ask the guy you bought it off of?

If it's an iPhone 4 and it uses SIM it's probably the GSM version for AT&T.

I have an iphone 5 i just bought off my friend i tried my dads tmobile sim but it did not seem to work, so how can i tell what carrier the phone is?