RG, here's an ol' farm trick:
Find yourself a sturdy tree that has a good hard pack around it (nothing your tires will spin on) and slowly put your front bumper tightly against it. Shift into first gear and let the vehicle return to idle. Now, ease out the clutch without touching the go pedal. If it stalls, your clutch is good & working correctly. If it sits there and idles while NOT spinning the tires, you've got a clutch that's about shot. If you are spinning the tires, find another tree!
I've used telephone poles when I couldn't get close enough to the trees. Cement bollards or pole protectors in some parking lots will also work in a pinch.
EDIT - If you are having problems with one gear in particular, you may have a syncro going out with that gear. Some trans shops will do a free inspect and let you know.