[KTL]

Guys that rebuild them like Kennedy will put them on the scale and bench-test the amount of poundage needed to operate it. If they find that the operational load is significantly diminished, the spring fingers are fatigued and it's done.

You also have to inspect the drive straps to make sure they're not cracked. These are the straps that connect the pressure/friction contact surface to the main housing and allow it to move but also stay aligned in the housing.

Most DIY people just look at the condition of the friction surface for heavy grooving and hot spots/discoloration. The clutch can certainly be a crap shoot if you're just looking at it. Ideally you'd like to be able to load it, so you can see how the pressure plate behaves. Some clutches are really old and if sitting for a long time can get quite rusty, since the pressure plate is iron.