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The easiest way is, get another SIM card from another carrier, then take out the existing SIM card from your phone and put the new SIM card on it. If your phone recognized this SIM card right away (meaning if you are able to make and receive calls from the new SIM card), then you have a, unlocked phone.
If you're asked for an unlock code or if you are unable to make and receive calls after putting new SIM card, that means you've a locked cell phone. To unlock the locked phone, you have to contact your carrier network and they will unlock it from there.
If you have any questions or would like any additional information, please don't hesitate to ask