So I have a line on an EFI 460 big block for $100. Guy says it's got no compression in one cylinder. I haven't yet spoken to him about it, only his wife. I don't know if he truly means ZERO compression, or just low compression.

My biggest concern is whether or not the block has been cracked. Is there any reasonable way to check for cracks in the block? Anything else I should be looking for>

 

No way to be certain other than a tear down and magnaflux, contrast fluid or other test to check. Of course, small cracks usually result in some reduced cylinder pressure and a difference cylinder to cylinder which may or may not be over the limit of 15% or so. A cracked, broken or dropped valve will result in zero compression. Burned valve would be very low pressure.

460s are like sand at the beach. Unless you have had real trouble finding one and would not object to the expense of sleeving a cylinder in addition to a complete rebuild, keep looking.