One of the beat ways to test to see if a clutch is worn out and slipping is to shift into top gear (5th on our cars) at a fairly low speed (about 25 to 30 MPH) and after the clutch pedal is full up, floor the throttle. A weak clutch will slip under this kind of strain. A really bad clutch will allow you to rev the engine at will at this speed.
I don't think this is what you are after though.
I sounds like you have already diagnosed your problem but don't realize it. You said that at times, especially when really hot, that it feels like the clutch is not fully disengaging when the pedal is on the floor. You are probably correct.
This can happen if you have air in the hydraulic clutch linkage. It can also happen if the fluid is really old. It can also happen if the clutch master or slave is getting weak. Before doing anything else, try bleeding the clutch.
To bleed the clutch, locate the bleed port on the top of the bell housing. It looks like a brake bleeder, but smaller. It helps to remove the air filter box and air intake tube if you have not done it before, but once you know where it is you can do it without. Just like bleeding the brakes, have someone pump the pedal (clutch pedal though) several times and while they are holding the pedal down, open the bleed port and allow some fluid to spurt out. Close the port and repeat several times (about 6 or 8 is usually enough) to clear any air in the system.
You really should change the clutch fluid from time to time as well. It is the same fluid as brake fluid. In fact, it shares the same resivoir as the brakes.
I change my brake fluid once a year and I do the clutch at the same time at least every other year. The clutch doesn't get as much heat so it is not quite as critical but it still should be changed to help the hydraulic parts last longer.
So see if bleeding the clutch or changing the clutch fluid helps with your problem. I'm betting that it will.
