Dial indicator

You could bolt a piece of flatbar accross the front of the bellhousing and use a piece of threaded rod mounted in the middle of that piece of steel to adjust it close to the input shaft for a reference point (if you can picture that) then slide the input shaft all the way out to it's farthest point and adjust the threaded rod against it then slide the shaft back in and check the clearance with a feeler gauge. Really wouldn't be too hard to do.