How do you tell if your engine is cracked form a blown headgasket?

Generally, if the headgasket is blown the rest of the engine is probably ok. The only way to determineif the block and/or head is cracked is to pull the head and have the machine shop "magnaflux" the head and/or block. Most often you can get by just machining the head (to make it flat again) then put it all back together. While it's apart it's common AND RECOMMENDED that you have the valves and guides reconditioned. It's relatively cheap while the head is off and can save problems later.