To start, the VIN on the dash should be on the fender tag, engine pan rail, transmission pad, and trunk gutter... if you're lucky it will have a build sheet with the VIN.
From there, check part#s and/or date codes on the radiator, carb, distributor, alternator, starter, rear axle, glass, hoses, wires, rims..... every damn part.
Numbers matching means many things to many people....
Some people have Holley carb, msd ignition and pump wheels and consider their car numbers matching because it has the original block. I think numbers matching should be much closer to assembly line correct.