It may be just a build-up of dust inside the clutch. You'll soon see when you take the cover off.
It's pretty self-evident if the plates are worn: the first couple of friction plates wear the most; you may find the friction material worn completely away in places. There's not that much thickness to start with, mind.
You can "hide" worn plates nearer the back of the pack by swapping them around. Make sure you keep the dished plain plate in the same relative position, though. It's near the back.
Check the friction plate tangs and the grooves in the outer drum for wear. But don't be too picky about this, as long as the clutch isn't dragging, or you'll be changing clutches every five minutes.
Get yourself a Haynes manual if you haven't got one already.