Perhaps if in a darkened area, drop a small light thru both spark plug
holes of each cylinder and see if there is any sign of cylinder wall
corrosion & that the valve openings seem normal. It would be better to
check in the rocker box covers though that is another level of
invasiveness. I agree jug removal only after a sale.

Reasonable oil consumption, clean plugs and a successful oil analysis
I'd think could confirm a reliable engine.

Assuming an O-320E2D with reasonable care, has there ever been any
engine more bulletproof?