others pretty much said it all,
-make sure theres no odd noises/feel in the shifter/clutch pedal
-make sure the clutch dont chatter, if it happens it makes the whole car shake bad, happens in my car.
-make sure when you shift it dont grind into gear or pop out, shift at different engine speeds and let clutch out faster or slower just to be sure everything is working fine.
-a test for clutch slippage would be put ebrake on shift into 4th gear and let clutch out like your pulling off from a stop light if it tries to stall clutch is good if the rpms' rise you know clutch is slipping for sure. can still test this by shifting into a higher gear at a slower speed then usual and press throttle pedal to the floor the rpm's and speed should rise at the same time if the rpm's rise and speed dont its slipping.
-just use common sense and listen/feel/smell for anything odd ask alot of questions, when was clutch replaced,timing belt replaced,fluids flushed and refilled,manual transmission fluid. ask questions like that and should be good. can also pull the top off of the clutch master cylinder resivour and check fluid color, should be pretty much a clear yellowish color.
hope the car hunt goes good for you...