Not sure if your phone is carrier-unlocked? Handy web site IMEI.info can tell you if you simply provide your IMEI number.
What is an IMEI number? IMEI stands for International Mobile Equipment Identity and is unique to your phone. You can find out what it is by dialling *#06# on any mobile. Alternatively, if you've got an iPhone, you can go into the Settings app and choose General -> About to find it. Once you've got the number, just enter it into the IMEI.info web site and you'll find out if you're unlocked or not.