You will smell it burning and your engine will slip as you pull away in first gear and while going uphill.
Slipping of the clutch and a peculiar smell are the first signs of a clutch that is worn out.
You have to look at the wear on the clutch and look for cracks. If the clutch does go out, the shifter will still move everywhere, but the transmission will not shift.
It is important to know the signs of a bad car part. If an AC clutch bearing going bad, the biggest indication would be the trouble driving with the AC on in the car.
The clutch will slip in higher gears first. Cruise about 40 mph in 4th gear and step on it, if the rpm's go up more then the speed your clutch is going
If you clutch and it either goes all the way in with no pressure there is a leak in your slave cylinder or the higher you have to clutch means your clutch is sliping and has to be replaced. If your doing a lot of power shifting i would suggest you get a racing clutch instead.
When fluid leaks out the back side where the plunger goes in, it is going out. Fluid would be seen from inside the cab, up above the clutch pedal.
clutch slave cylinder.
There are various ways through which you can know when your clutch is bad. When you are having troubles shifting gears, this might be a clear indication of a clutch problems.
a failing clutch will slip,in 4th or 5th if you floor it the engine will speed up without the car going faster
the clutch is not adjustable but you can take up some pedal free-play by going under the dash and locating the adjustable rod going from the clutch pedal to the master cyl
it may bind when using the clutch, it will squeal when not using the clutch, and stop when you do. You may also feel or hear a constant grinding like noise, this is the bearings going bad.
You should not attempt to replace the clutch yourself unless you have considerable mechanical skills. In this case, the problem is most likely that the spring holding the clutch pedal in place is broken.