i knew mine went out when i got on the highway and tried to give it gas in 4th gear.
couldn't get it over 55 without it slipping.
so now almost $300 later its going to be fixed, thats a clutch kit, slave and getting the flywheel machined along with a shim.
if you get a clutch i would get one from http://myworld.ebay.com/clutchcityonline
for the price and the quality of the parts you cant go wrong.
also you can tell by how high the clutch pedal is sitting and how long it takes to "grab" after you let up off the pedal.
mine went out after 135k.