Know the three kinds of problem
A counterfeit is a coin made to deceive, often cast or struck from fake dies. An altered coin is a genuine coin changed to look like a rarer one, for example a mint mark added or a date modified. A replica or copy is made as a souvenir and may be correctly marked as a copy, or may not be.
Most beginners meet counterfeits through online marketplaces and unfamiliar sellers, and altered coins through raw rare dates sold at a price that looks too good.
Start with the price
The most useful first test costs nothing. If a scarce coin is offered far below recent sold prices for the same grade, ask why. Genuine bargains exist but are rare, and rarity attracts fakers.
Search the item in the catalogue, read the sold results for the exact grade, and compare. The deal check tool will tell you whether an asking price falls below the fair range, and below the range is a reason to look harder, not to hurry.
Weight, size and magnetism
Genuine coins follow published weights and diameters. A small digital scale accurate to 0.01 g and a pair of calipers are cheap and catch many cast fakes. Compare your readings to the mint's published specifications; allow for wear, which lowers the weight slightly on circulated coins.
Many precious metal coins are not magnetic, so a coin that jumps to a magnet is suspicious. A non-magnetic fake is still possible, so use this test only to rule coins out, never to rule them in.
Look at the surfaces and the details
Struck coins have crisp, directional flow lines in the field and sharp lettering. Cast copies often show a grainy or porous surface, soft edges on the lettering, and tiny round bubbles. Check the rims and the edge: a visible seam, filed marks, or reeding that is uneven can mean a fake.
Compare against a certified example photographed at the same angle. Look at the shape of the date digits, the spacing of the letters, and the position and style of the mint mark. Altered mint marks often sit slightly off, with different surface texture around them.
Be careful with slabs
Counterfeit holders and genuine coins in re-sealed holders both exist. Check that the certificate number validates on the grading company's own site, that the label font and colours match known genuine examples, and that the holder shows no signs of having been opened.
If you are spending serious money, ask the seller about returns before you buy and keep all communication.
If you think you have bought a fake
Stop and keep the packaging and listing. Contact the seller and the marketplace or card issuer straight away, since protection windows are limited. Hold the coin by its edges and photograph it clearly.
If you are unsure, a recognised dealer or a grading service can authenticate it for a fee. Do not try to sell a coin you believe is fake, even to recover your money.