The check valve would be in the master cylinder. If it's shot the clutch fluid lever will go up as the pedal is pressed (after a couple centimeters). Have someone watch the fluid level while you pump the pedal all the way.
I Google'd for a decent depiction about how they typically work:
http://www.4g63turbo.com/tech/eclips...djustment.html
Actually, I might be wrong about the fluid level thing... I've always just rebuilt or replaced the mast and slave under such circumstances.