"Q: I just upgraded my iPhone but now cannot activate it because I don't know the carrier that it's locked to. How can I find out which SIM/carrier that this iPhone is locked to?
- Sly
A: Unfortunately, there's no straightforward way to do this as it's actually based only on your iPhone's International Mobile Station Equipment Identity number, or IMEI. Unlike most other cellular phones, Apple doesn't allow any carrier-specific branding on either the hardware or software, so there's nothing obvious you would find on an iPhone that would indicate which carrier it came from.
If you know anybody who uses an iPhone on an alternate network that your iPhone might possibly be locked to, you could simply try popping in their SIM card and see what happens. This is obviously a trial-and-error method, and may not be practical if you've purchased the iPhone from an online site, since it could conceivably be locked to any carrier in the world.
Another option would be to try visiting a local Apple Store, where they might be able to look up your iPhone's IMEI number and let you know which carrier it is locked to. Apple does have this information in a database"
ilounge.com