Usually, if a phone is locked, when you boot it up, it will start with a flash screen showing a logo of the network (like AT&T logo, or a T-mobile logo, etc).
You can look up your IMEI #
To get that number, Dial *#06# on your device to find your IMEI. Then use an IMEI checker service.
You can also go to Settings
Then go to About
Look through the Device Information and check what is listed under Carrier
On my 950XL, it shows Carrier: NAM CV (That means it is an unbranded phone, not tied to any carrier. If it was a phone I got from a carrier, their name would be there, like AT&T. The NAM CV is North America country variant. The region it was made for)
On my Nokia 1020, it shows Carrier: AT&T which means it is locked to AT&T. If I went to another provider, I could get it unlocked by my new carrier.
I think the easiest way is to go to the cell provider you want to use. They can easily figure out what you need to do and if it needs to be unlocked then they can help you out.