If you're on the accelerator at the same time it may be causing it. Best bet is to drive along, dip the clutch then quickly dump it (maintaining a constant accelerator pedal) and if the revs jump then something's not right. Do it with about 2500rpm going.
What that does is dump a load of torque onto the clutch, which isn't something you should do regularly if you want it to live a long, peaceful life.
