Get the car on a lift and check the undercarriage. Observe the floorpans for odd seams that appear out of place, which would indicate an illegitimate repair. Look at the torque boxes (if equipped). Knock with your knuckles on various locations, listening to the sound as you tap. If there is fiberglass or body filler in structural members, like torque boxes, rocker panels, and floorpans, politely decline and walk away. Ditto for poorly patched floorpans and the like. Heavy amounts of undercoating typically mean someone is trying to hide rust or a poor rust repair. There's nothing wrong with rust repair, if performed properly. We stress repairs performed at the factory seams--not halfway. Poor structural repair is not only unsightly, it is also unsafe.
A prospective buy's inspection begins with body and structure, but it includes so much more. Is the vehicle as represented? Is it an "original, matching numbers" car? This is where knowing engine, transmission, and rear-axle casting numbers and date codes is so important. If you're looking at a '70 Boss 302, is the Boss 302 engine the real thing? Is it a service replacement? Is it a Boss 302 block? Are the heads Boss 302 castings? Believe it or not, you could actually fake a Boss 302 engine using a 289/302 block and 351 Cleveland heads. On the surface, the untrained eye wouldn't know the difference. Except for the VIN, the engine code being most important, you could fake a Boss 302 Mustang. This is where doing your homework is so important.
But "matching numbers" count outside of the exotics like the Boss 302, Boss 429, and Shelbys. Mustangs prior to 1968 don't have the VIN stamped into the engine block (except 289 High Performance models). This is where you must be savvy about casting numbers and date codes, which cannot be faked, so if they are ground off, walk away. A good rule to remember with Ford castings is this: The actual date is cast into the iron or aluminum, while the date of component manufacture is stamped into the casting.
If you are buying a '65 Mustang, ideally, you'll want the engine to match the VIN. Using a Mustang warranty plate/certification-sticker decoder, examine the VIN and the vehicle makeup. If the VIN says the vehicle was equipped with a C-code 289-2V V-8, does the engine underhood reflect this? Is it the original 289 engine, or is it a replacement 302 that looks like a 289? Ask this question whenever the seller says "all original."
Perhaps the vehicle you're considering is a '69 Mach 1 with an R-code 428 Cobra Jet. How do you know it is a Cobra Jet? How do you know it isn't a 390 from a '67 Galaxie made to look like a 428 CJ? This is why close inspection of all engine castings is critical. When shopping, be armed with the right publications, some of which we list at the end of this article.