I'd say its about time to change it out for an aftermarket SBC or something to that effect. Your lucky to get that many miles out of the factory clutch. I only got 80,000 out of mine when the throw out bearing seized and got me stranded out in the middle of nowhere. Changed it out to a Centerforce Dual Friction since I didn't have the $$ for an SBC.
About the inspection, without dropping the transmission, I don't see a way of checking the wear without doing that. It's just one of those things. I suppose you could try to look at it through the grove at the bottom of the bellhousing with a flashlight, but even that is almost impossible to get a good look through.