How do i know if my iPhone 6s is unlocked?

And if the above test fails...

How to unlock your iPhone for use with a different carrier - Apple Support

The easiest way to check whether your iPhone is carrier locked/unlocked is to install another carrier's working SIM card. If you know someone who uses a different carrier than you do then borrow that person's SIM card (Nano Size) and install it in your iPhone. If your iPhone is then able to make and receive calls including mobile data then you know that your iPhone is carrier unlocked. If you cannot make calls or access cellular service then you now that your iPhone is locked.

Axel F.