I'm starting to suspect that my 2002 S40 body and engine may not have started life together, the hood has several spots that look like tree sap and under the hood was patches of leaves on both sides like maybe this car might have sat for a long period of time. where do I check on the engine and the body to see if the numbers match?

 

There should be a VIN visible through the windshield somewhere along the bottom edge of the windshield.

I believe that the VIN can also be referenced on the engine itself to the left of the battery, below the throttle body, just up from the transmission oil dipstick.

 

It's been 12 years since that car rolled off the lot! A lot can happen in 12 years.

You can check all the various VIN locations (including, AFIK, the vehicle setup dialog in the "infotainment system") to see if they all match. If they don't, that would mean panel replacement. I believe every panel is tagged somewhere.

The motor ser#, however, is separate. I believe Volvo has a database of VIN / motor# pairs.

You can also Google volvo vin location for more info. This YOUtube vid gave me new info: