To check if your clutch is worn out ('slipping') - on level road, pull your handbrake on firmly, engage first/low gear, rev to around 1200 rpm and let the clutch slowly and smoothly out.
If your handbrake is holding firmly... as the clutch reaches its biting point, revs should immediately begin to dip/fall as your car appears to be 'straining at the leash'.
If you continue to let the clutch out, the engine should stall/stop. If unsure, repeat the test (just once or twice.)
If revs don't drop markedly to the point of the engine stalling, it is most likely your clutch is 'slipping' and needs a mechanic's attention.