I found a really good deal for a Cummins engine, complete, needs rebuilt, but because of the low price i am skeptical that the block and head are free of cracks, how can i test the block and head quick and on the fly before i buy it?

Pulling an engine out of an salvage yard (Big Block Chevy) I understand how to do the test. assuming the engine is warm/hot But What are numbers i should look out for? What is considered red flag and what is not? What is are good numbers? and if the cylinder(s) has low numbers does that...