10.2 vs. 9.8 block

I'm looking to buy a used 454 block to build a race motor....496. I see a lot of blocks for sale at reasonable prices, but I have very little experience and knowledge of big blocks....I've always been a small block guy. Is there an easy way to determine what you are looking at? What is a quick check to see if a block is the 9.8 or 10.2? I assume you can just measure the deck height, but I'm not sure how you would get an accurate measurement. Of course, if you have one of each, side by side, it would probably be easy to determine, but that is not usually the case. Thanks.

The upper water pump bolt holes on the 9.8 deck blocks are close to the deck---on 10.2 blocks they are about 1/2 inch away---

Thanks for that info.....