Not exactly where I was going. Lol.
I was thinking more like, if you're gonna buy an engine that has been sitting for a while, be prepared to tear it apart and at least reseal it.

Pull the fuel rail off and see what the spray looks like.

If you don't mind the mess, take the value cover off and crank the engine (don't start it) there should be ample oil flinging everywhere. If there is any doubt if it is enough, then it isn't.
You can take it a step further and pull the upper timing cover off and repeat that messy process. Should give you a good look at how the timing again moves around.

If you still don't have any clues, then I'd be pulling the head off to see what the insides look like.
I'd be looking for pistons I could wiggle with my fingers. Or valves that aren't seating.

Honestly, at this stage of the game, I'd just bite it and pull it all apart.

You could drop the oil pan and see how much play is in the rods to rule out rod knock.