When I first heard about counterfeit classic cars, I thought it was in reference to collectible classic car toys. Was I ever wrong. We're talking about the real deal here, actual classic cars, from muscle cars to sportsters to luxury cars. It's become enough of an issue that TUV Rheinland Group, "a global provider of technical, safety, and certification services" (source: Wikipedia), employs a vintage car consultant who specializes in authentication. And that's not all, there are even attorneys who specialize in collector car cases for those who have been defrauded for large amounts of money.
As the value of some classic cars increases quickly (one of the highest auction prices realized was $32.1 million for a 1954 Mercedes F-1 in 2013), so does the number of fakes. Some cars are so well faked that even the experts have to go over them inch by inch, following their gut instinct that something just isn't right, the same way most collectors do, while others are more obvious to the trained eye. Among the high-end cars that have been faked are Aston Martin, Jaguar, MG, Ferrari, BMW 328 Roadster, and Shelby Cobra, among others.
Recognizing a faked or counterfeited car isn't like recognizing a fake small collectible. Many times the questionable areas are hidden or tiny within a sea of technology, and you need an extremely in-depth knowledge of the particular car to spot it. Classifying a car as a fake can be a tricky thing because certain restorations or replacements are allowed, but the key is whether the restoration is allowable and, more importantly, whether it's been disclosed to the buyer. Replica cars, adapted cars and the like are only an issue when they're meant to deceive and sold as something other than what they really are.
It gets pretty complicated. There are legitimate classic car replicas that sell for far less than their authentic counterparts, and there's nothing wrong with buying or selling one of these, as long as it's priced accordingly and the buyer knows the car is a replica. Problems arise only if someone claims a replica is an authentic classic car and tries to sell it for the high price of an original, which has happened many times.
With that as a foundation, let's take a look at some of the scams that have been discovered to get an idea of the type of caution that needs to be taken when considering such a large investment.
One of the most commonly faked cars is the Pontiac GTO. More than simply replacing smaller parts that may not be within permitted parameters, faked classic GTO muscle cars have actually been vintage Lemans bodies with GTO engines, made to look even more convincing with GTO emblems. These types of fakes, done with other models too, are frequently referred to as "clones."
You'd think that a car's chassis number that matches official records would have to be an authentic car, but, as the old song says, "it ain't necessarily so." That was the situation with a 1967 Ferrari 275 GTB/4 that was in a serious accident that burned it completely. But an "enterprising" fraudster who knew the value of the Ferrari would be over $3 million took the liberty of extensively modifying a 330 GT chassis, used the chassis number of the original burned-out car and stamped an unnumbered engine block with the number from the original 275 GTB/4. Only the fact that there were wrong component numbers that didn't belong together revealed the car's true secret.
When a limited number of cars are made by a manufacturer and you discover there are many more for sale, your warning light should go on. This was the case with the MG K3 racecar made in the 1930s. Only 33 of these cars were produced, yet today there are over 100 of them for sale. The additional cars aren't authorized replicas; they're flat-out fakes.
Those who craft fake cars use some of the same techniques as fraudsters of smaller collectibles: they leave reproduction frames outdoors in extreme conditions to give them an aged look; they use period screws and washers to connect reproduction parts; they use reprinted warranty books and forge signatures, service dates and stamps. Perfectly worn leather seats may have been made from salvaged old theater seats.
Years ago, Carroll Shelby sued a company for making replicas of Shelby cars. He lost the case, and the ruling stated that there's nothing illegal about another company making a car that looks like a Shelby, as long as they don't use the Shelby name on it. And as long as replicas are being made and sold as replicas, with prices far lower than the originals, everything is as it should be. Car lovers who can't meet the price of an original vintage Shelby can fulfill their dreams with a lookalike that they can afford, but sometimes these cars are resold down the line by someone who makes all the changes that mimic an original and tries to sell the replica as an authentic Shelby with a price tag to match.
So if you're in the market for one of these truly grand cars, do your homework by checking car identity and ownership records online and join classic car collectors' clubs. Their members have a wealth of information and are more than willing to help with your questions.

At A Glance
Possible signs of a fake or clone:
1. No traceable history of the car or a gap in its history
2. Wrong component numbers
3. More than one car with the same ID number

Reference books, websites, and collecting groups are the best ways to keep from being fooled by fakes. Here are a few: Classic Car Club of America (http://www.classiccarclub.org), Classic Car Database (http://www.classiccardatabase.com/clubs.php).