How to tell if 4wheel drive is working on a 1998 chevy blazer

Depends on the system.
It is impossible to drive a true 4 wheel drive vehicle on the road due to wind-up between the axles so first a unidirectional clutch was fitted into the front drive to prevent wind-up but the main problem of one wheel losing traction and the whole vehicle becoming stuck remained.

Over the decades differential locks and/or limited slip differentials have been fitted to overcome the basic problems.

In more modern times refinements, often to cut costs, such as viscous couplings have been used to produce automatic or part time 4 wheel drive vehicles and the development of abs and later more powerful computers added traction control systems.

Jacking each wheel in turn from the ground should reveal whether drive is being transmitted to it. When there is a centre differential lock or it is of the limited slip variety, using a jack with good wheels and on smooth ground lift first the front and then the rear and attempt to drive. If it is working in each case the vehicle will move and should roll the jack along the floor.

The front and rear differential locks can be tested in a similar fashion but 3 wheels will need to be raised.