I'd start with an electrical diagram of the car, and try to identify the connector that activates the compressor clutch (by the colors of the wires). I think it should be right on top of the passenger-side of the compressor.
Then, using a voltmeter, I'd check if the connector gets DC voltage when the A/C is turned on. If it doesn't, the problem is with the wiring. If it does, the problem is either with the connector itself, or the compressor clutch. I guess that typically it'd be the connector in this situation. Check the connector to see if anything'* broken or bent.
Another possiblity is that the compressor is not engaged because of some sensor indicating that there'* enough pressure in the system already, and therefore it isn't needed. Then of course it won't get voltage at all at the clutch. Again, I'd check all of the connectors going to the compressor (I think there are 3 - I recently went through the process of replacing the engine in my '92, though a '98 may be different).
Hope that helps