Netgear Nighthawk mr1100 how do I know if it is unlocked or locked to a carrier the SKU is 6420B

[celler]

If you are in the US, there is a good chance it is ATT.  You can go here:

https://www.att.com/deviceunlock/

and check to see if it is an ATT device and if it is unlocked. If you get past the first page without an error, then it is unlocked on ATTs system.