What to look for in U-pull Magnum?

Make sure it turns over freely.

I had a magnum engine pulled in a yard, took it all the way home, turned the crankshaft over to find that #1 piston did not move at all..crank or rod was blown. Luckily they had a good return policy and exchanged it for another.

And yes you usually can see the crack between the valves. Bring some scotchbrite, flashlight, wd40, and lots of rags. It's ,most likely cracked anyways, I'd say 9 out of 10.