How can you find which carrier a Samsung Galaxy S3 (GT-I9300) is locked to? I bought a S3 and it came locked to an unknown carrier. All the unlock services require the information of which carrier the phone is locked to give me an unlock code. So, how can I find that out?
Depending on where you are in the world, and which carrier you're with, your phone will have a unique model.
You can find your phone's model number by going to Settings > About device and it'll be around the middle of the page.

North American models

Carrier Name                Model Identifier

AT&amp;T (US)                   SGH-I747
Bell Mobility (CAN)         SGH-I747
Moblicity (CAN)             SGH-T999
Rogers Wireless (CAN)       SGH-I747
SaskTel (CAN)               SGH-I747
Sprint (US)                 SPH-L710
T-Mobile (US)               SGH-T999
Telus Mobility (CAN)        SGH-I747
U.S. Cellular (US)          SCH-R530
Verizon (US)                SGH-I535
Videotron (CAN)             SGH-T999
Wind Mobile (CAN)           SGH-T999