Cylindrical square from a lathe tailstock.

[DIYSwede]

I'd expect wrist/ gudgeon pins being very square by nature [to length as well], with expectation retaining rings or buttons keep them located. Lapping ends won't degrade hardness, even case-hardened.
Just visualized über simple lap fixture. The proper connecting rod, provided crank and pin end are same width, not twisted of course. Certainly square by design. With a hand motor, relieving [grinding] bottom into 3 pads would be easy.

Sure - but for smallish ones used for squaring purposes on, say a face plate or a mill I'll need them cut in half, thus the lost hardening.
Here I simply assume they're not thru-hardened by the case hardening. I'll get a matched pair from each pin, though...
After initial checking for true roundness, I'll probably make an adjustable soft aluminium lap for the diameter, as per Tony's design-
carefully putting my handle so it won't self-tighten on the pin and lock on me.

For checking their axial trueness: just a simple setup as per Franks suggestion:
Clean and drop three of them on two long axles (held together on the surface plate), both axles grooved at the centre pin's ends.
First one against a ball bearing end stop, a mumeter indicator at the third's farthest end.
Turn just the middle pin and check indicator, tape 1st and 3rd pin lightly to one axle if needed to avoid their rotation.
Clean, flip, change and swap ends 'til all surfaces have been checked - 6 runs would suffice.

Then, if found necessary: lap ends thru finer grits in the same way but swap indicator for spring loaded ball bearing.
Rinse and repeat 'til satisfactory trueness is reached. Think I'll stick with circular end surfaces - not 3 pads.
Then cut, grind, relieve and lap the fresh (unhardened ends) of the short stubby ones. Mark for pairing.
Did I say: -"Clean meticulously between each and every step above"?