The above suggestions will work.

Or you can have a buddy follow you onto a dirt road. Stop the Jeep and then tromp on it vigorously. Your buddy should see all 4 tires spin (or at least one front tire and one rear tire).

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.