Doing H/C/I how can I make sure my block is good?

Its called a straight edge. We used them at school to check for deck straightness. There are two kinds though. A flat one with a machined surface and a round one. The round ones are more accurate though. Get yourself one and a Feeler Gauge while your at it. But you need to look the factory specs to find out where to position the straight edge on the deck of the block. You take the feeler gauge and see which one(if any) will fit under the straight edge.The biggest one that will fit under the straight edge is the amount of warpage you may have. Its usually a machine shop type thing though.