The cardback test
Flip the card over. The cardback should have a sharp, clean Pokémon TCG logo with the blue gradient running consistently across the swirl. Fakes commonly have a slightly washed-out blue, blurry text under the logo, or visible halftone dots if you look at it under magnification. The cardback test catches roughly 80% of casual counterfeits before you need to look at the front.
The light test
Hold the card up to a bright light. Real Pokémon cards have a black or dark gray inner layer between the front and back, which blocks most light. Counterfeits frequently skip this layer and the card glows through. This test does not catch high-quality fakes but it ruins the cheap ones immediately.
The rip test (only as a last resort)
A real Pokémon card has a paper interior that tears in a characteristic way. Clean, with a faintly fibrous edge. Counterfeits printed on plastic or low-quality cardstock tear differently. This test destroys the card so it is for confirming a fake you already strongly suspect.
The font test
The Pokémon TCG uses very specific fonts on its cards, and counterfeits often miss the kerning and weight on subtle parts of the card. The most common fail point is the energy symbols at the top right (the small colored circles indicating attack costs). On fakes, the colors are typically less saturated and the circle outlines are slightly fuzzy.
Modern fakes: the proxy market
A separate category exists: high-quality "proxy" cards explicitly sold as replicas. These are not legal counterfeits in the sense that they are not sold as authentic, but they are still illegal Pokémon trademark infringements. The diagnostic is the legal text on the card bottom. Real cards have full Pokémon Company copyright text; proxies often have shortened or missing text.
Slabbed card authentication
A graded slab is a useful but not infallible authentication. Verify the certification number on the grader's website. PSA, BGS, CGC, and SGC all maintain public databases keyed to the cert number on the slab. The cert lookup tells you the card, the set, the grade, and confirms the slab is real. Counterfeit slabs do exist but are rare for cards above $200 because the underlying card needs to look genuine on the cert listing too.