is there a certan way the imput shaft wears, or something?
i have one in my garage with unknown mileage and really dont want to put it in and have it be just as bad as my old one. but by the same token i dont want to shell out 800 bucks on a rebuild if i have a good one in the garage.

 

Good indication is its engagement.

Won't tell you the mileage per-say, since there's really no actual way of telling (short of either owning the car since new, or rebuilding the trans and recording the miles on the car when it was performed), but will give you a decent idea about the abuse or lack thereof the trans has had.

Run it through the shift pattern and see how it feels.

 

i pulled the shifter out and the bushings (looked stock) were both perfect, obviosly old, but still complete, not broken cracked or chipped at all. the spring on the 1st and 2nd gear side is broken though, but i cant recall ever seening an fc with a good one

 

No way to be sure unless you take it apart and start measuring. Mileage and driver are key factors. My buddy went through three trannies, and I'm convinced it was the driver.