How do you diagnose a used engine?

I am thinking of buying a used engine from a junkyard or somewhere like that. I would like to know how to diagnose an engine while it is out of the vehical. I dont want to replace a bad engine with another bad engine. The price difference between used and rebuilt is hard to ignore.

its hard to tell the condition of a used engine. i mean you can take off the oil pan and play with each rod and see if theres rod knock, pull the spark plugs and see the condition of them

you can pull the head and get a good view but thats alittle more work