If you see in the ad:
"All original"...car should have 99% of the same parts that were on the car when originally purchased, including paint.
"Numbers Matching"...VIN's were stamped into the engine and tranny and should match the car's VIN. Obviously paint, trim, and interior codes should "match" what's on the cowl tag. Date codes on parts such as the engine, tranny, alternator, rearend, heads, manifolds, etc, should be recent to the build date of the car. Car should have few if any "replacement" parts (i.e...a Raybestos Master cylinder)
"Correct Numbers" or "Period Matching" etc, will indicate the car has parts from other like models from the same year, but the parts aren't original to the car...maybe the date codes are off, but a 1967 Muncie was replaced with a 1967 Muncie...a 1967 327 was replaced with a 1967 327, etc.
This is rough and it's a moving target with sellers...be careful. In the end ask yourself, "what's the car worth to me?" An all original sweet running well optioned 6 banger with a powerglide ain't worth much to me. But a SS with replacement parts, few options, different engine block, aftermarket exhaust, torn seats, saggy doors, sqeaks and rattles, etc,...Now, that gets my attention.