The first thing I do is scope out the VIN pad for the VIN number/letter code that is right above the pass. side top water pump bolt then later see what my books say what the block started life as?
You just have to look at other things to tell what it is if the block has been decked and the VIN letter code is gone.
It's the letters that decode it!!
Otherwise look at the damper if a SB b/c a 400 has a scalloped one with a bump whereas a 350 on down has a round one unless it is an internally balanced 400 motor running a stock 350 damper or has an aftermarket damper on it?
Same with the 427 on down BB's, but here you have to finger the inside back of the balancer to check for the externally balanced 454's dog-knot vs all the others being internally balanced and smooth back in there!