The way to know for sure if the clutch is slipping is to step on the pedal when the RPM is low. My clutch needed changing and this is how I knew.
On the interstate when driving at around 100-110km/h on 5th gear (RPM should be a little below 2000), just floor the pedal and see what happens. Does the car start accelerating (even if it is slow), or do the RPMs increase fast without the speed changing and then lower suddenly, with the car starting to accelerate. If it is the latter, then the clutch needs changing.