How to find out what carrier my Iphone 6 was bought from?

I have recently received an Iphone 6 but how do I find out what carrier it was bought from. The person I got it off has no idea and neither do I but I need to know in order for it to be unlocked. I've tried using my O2 sim inside it but it comes up with SIM Not Valid and the message that comes up when it's locked which rules out O2 but I need to find out which carrier my phone was bought from so I can unlock the Iphone and use my O2 sim in it.

Check the About Phone/Device in your setting options. Or find one of the IMEI info checkers online and type in the IMEI.

First get the serial number on the phone and try that way. I know Verizon doesn't activate phone's if you got it off someone unless there's a serial number. Reason for that is because with the serial number they can see if its stolen. It seems fishy to me that they gave you an iphone 6 and did not know what carrier its from. If it turns out its a stolen phone no carrier will acitivate it for you