A good test to know if your clutch is slipping is find a steep uphill, have a slow rolling start, put it in 3rd/4th gear and put your foot down.
if you hear your revs going up and your not actually going anywhere then your clutch is most likely slipping.
but if you floor it and find yourself doing 90kph, then its probably time to start thinking about braking.
but hey, at least your clutch isnt slipping