pull one spark plug and put a volumn tester on it. You can probably borrow one from a mechanic friend. An 865 will puff 432 and a 790 will puff 396. usually 05 and earlier is a 790 and 06 and later are 865s. I perfer the 790 myself they produce a greater torque to horse power ratio so its easier to hot rod it. That is because the cam as mentioned by Dr Pc above. Plus if you decide to go big bore the less expensive 904 is a really nice step up.

I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago. Edgar Allan Poe