looking arround at the local junkyards and have spotted many 350's and looked as if they were pretty fresh rebuilds and was wondering how to know for sure if i decided to go ahead and pull one whats some of the most important tell tell signs that would say this block is no good? one block inparticular looked as if it had no oil build up in the lifter valley, no lip to catch on any of the piston walls, and just looked like maybe it was possibly rebuilt. but how can i know its not a dud?  

well one way would be get u a socket and a long breaker bar and stick on one of the bolts inside the bottom pully/harmonic balancer and turn the crank over the see if there is any internal crank/rod/piston damage. usally if u cant turn it over theres obviosly somthing wrong inside the block and should probally just move on to the next one. if able to turn look at the piston walls for any kind of damage. if u have block out of engine bay look for any external cracks/damage. hpoe this helps.