How can you find which carrier a Samsung Galaxy S3 (GT-I9300) is locked to?
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