I've always tested the 4x4 by putting it in 4hi, and turning the wheel all the way one direction, then just put it in drive and let off the brakes, if it rolls a tiny bit then stops, and acts like it wants to hop if you lightly tap the gas then its working, if it keeps rolling then its not engaging. Just don't stomp on the gas or drive it anywhere on dry pavement in 4hi.

 

I wouldn't suggest doing what compactc9 suggested just in case you break it before you own it. If you truly wanna test it, find some gravel and put it in 4hi. You will know if it's engaged or not. Or just ask if the owner mines proving the 4x4 works that way you don't have to worry about breaking his/her stuff

 

 

I saw some misinformation, NEVER tow a D in the t-case neutral position alone. Yes that will prevent problems in the trans but the t-case still has bearings that will not have oil pumped to them without the input of the t-case receiving power. Your 3 options when towing are a flatbed, pull the driveline to the wheels that are on the ground, or have wheel dollys under the wheels that would be in contact with the road. Just thought I'd throw that out there even though it is off topic nobody wants a toasted t-case.

 

+1 good advice