The air conditioner compressor clutch will engage off and on during an engine warm up cycle, so you could watch for this while it's warming up. Also, if its some what warm outside you could turn on the air conditioner and watch for the clutch to engage. The bearings in the large pulley on the compressor started to come apart on my 91 4.0 and made the engine vibrate and caused a screaching noise. I bypassed the compressor pulley with a belt for an engine without AC and it was all good after that. You should be able to do the same thing.