I would actually verify that fridge is level when floor is level. If you get that verified, then all you have to worry about is the floor level.
I open the door and place small torpedo level just inside the door. Works pretty well.
Just recently decided to start using my front storage compartment floor. Same floor, and works pretty well too. Except when I don't have a clear spot to put the level on. Which is pretty much always.
I've been known to use the torpedo level on the tongue for front to back level, and seems pretty accurate. But side to side level I generally open the door to check.
In other words, try a bunch of different methods and see what works for you.
