If you can successfully make a call with another carrier's SIM card in your phone, your phone is unlocked; however, if you can't, the phone is carrier locked and you'll need to talk to your carrier about unlocking it.

Before getting a new SIM card, figure out which size of SIM card your phone will use. You can consult the phone's manual, or you can research the phone model online.
 While this process will vary from phone to phone, doing so will usually entail pressing and holding your phone's Power button, then pressing a button or sliding a switch to turn off the phone., If your phone has a case on it, first remove the case. You'll most likely have to consult your phone's manual or look online to see where the SIM slot is if you don't already know.

On iPhone, the SIM slot is either on the right side of the phone's casing (iPhone 4 and up) or on top of the case.
Android phones vary their SIM slot locations, but you'll often find the slot on a side of the casing or under the battery cover.
 On some phones, you'll simply need to pull the card out; on others (e.g., iPhones), you'll need a paper clip or SIM removal tool to insert into a small hole next to the SIM slot., Be sure to reference the positioning of the original SIM card in order to ensure that you don't improperly insert the card., You'll do this by pressing and holding your phone's Power button., Once again, this process will differ from phone to phone: open your phone's calling app, type in a number, and press the "dial" button. If the phone call goes through, your phone is unlocked and should be able to accept any hardware-supported SIM cards from other carriers.

If you are unable to make the call and you're certain that the number you're calling is valid, your phone is locked.
Buy a different carrier's SIM card or borrow one.
Power down your phone.
Locate your phone's SIM slot.
Remove the SIM card from its slot.
Place the other SIM card in the phone.
Turn the phone back on.
Try to make a call.
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