The main thing is something you can't look for: do the shutter and aperture mechanisms still work properly?
The shutterspeeds, especially the long ones, tend to become slow, due to filthy, possibly hardened (due to lack of use) grease.
Focussing can get stiff. The PC-flash terminal may have gone bad (if so, it's a loose contact inside, which can be fixed, but probably needs parts that have to come from another, donor lens). And (related to the shutter thingy) the self timer/X-M synchronisation selector may notwork anymore.
Apart from that, if the glass is clear and clean, and the barrel doesn't look like it has gone through a couple of nuclear wars, there's not much.
But that's about the C versions.
The later CF versions (and the later CFi/CFE) also can have a sticky shutter. The EV-coupling button may have gone. The DOF-preview lever may have become dislodged (not working the aperture anymore). And the clip ring behind the rubber on the focussing ring may have broken and gone awol.
Nothing that can't be fixed. But better to not have to have it fixed, of course.