If you purchased your iPhone directly from Apple, it is most likely unlocked. However, if you bought your iPhone from your carrier, it is probably locked to that specific network. The best way to make sure is to call your carrier and ask. They may ask for your IMEI, which can be found printed on your iPhone’s SIM tray or by going to Settings > General > About. After providing your device information, your carrier will be able to tell you whether your iPhone is already unlocked or if you are eligible for a free device unlock.