Most Atari carts are worth a few dollars, which makes faking them pointless. The exceptions are severe: Air Raid, Birthday Mania, Red Sea Crossing, Gamma-Attack, Karate (Ultravision), and the various rarities from Panda, Zimag, and CommaVid. These sell for four and five figures, and they are faked.

What to check

• Label printing above all. Atari-era labels used specific printing processes and paper stocks. Modern reproductions almost always show inkjet dot patterns under a loupe. Genuine labels also age, so expect some yellowing, edge wear, and adhesive bleed consistent with the rest of the cartridge.
• Shell mold marks. Atari and its licensees each used distinct shells. Air Raid famously used a blue T-handle shell unlike anything else in the library, which is why the fakes concentrate on getting that one shape right. Compare mold seams and interior ribbing to documented examples.
• Board and chips. Genuine 2600 boards use period-correct mask ROMs with date codes in the late 1970s to mid 1980s. A 2600 cart containing a modern EPROM in a socket is a homebrew or repro, not a rarity.
• Screws. Standard Phillips throughout the Atari line, so screw type is not a useful tell here. Go straight to labels and boards.
• Provenance. For any Atari title in the four-figure range, provenance and expert authentication matter more than physical inspection. There are only single-digit known copies of some of these. Buy from established collectors, not from a random listing.

Note on 5200 and 7800: the same logic applies, with even fewer targets worth faking. The 7800's Sentinel and PAL-exclusive titles occasionally attract attention.