Bad clutch job?

[nearmiss]

I've been wondering if my 96LT4 had a bad clutch job at some point in its life due to some engine vibration above 4K rpm. I'm the 3rd or 4th owner and the car has 85K miles. I understand that the engine balance is done externally and if the new clutch/flywheel isn't balanced like the old, then there could be a vibe problem. Does anyone know how to find out for sure if this could be the problem and if it is, how do you correct it?

thanks!
nearmiss