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The only magnetic US coins you will find right now are 1943 steel cents, though the US Mint is investigating steel as a.

To test for it, simply get a magnet (an ordinary fridge magnet will do) and see if it sticks. There may be other kinds of electrically conducting flakes in your sample, but gold is one of the better electrical conductors, and this level of separation may already be a big step forwards. 'Magnetic' coins are coins that will stick to a magnet, because theyre made of either steel or pure nickel. These eddy currents will oppose the change in magnetic field, and the net effect will be to repel electrically conducting flakes from the alternating magnetic field. If the magnetic field is both non-uniform and alternates in time, then the magnetic field will generate eddy currents in the electrically conductive gold flakes. There is a version of this which may be more successful, though.

However, it turns out gold is weakly diamagnetic, so that magnetic tip would actually repel it a tiny bit. So making the magnet with a sharp tip might help pick up small paramagnetic objects. Actually, what’s important is not the strength of the magnet but how non-uniform the field is.

If the once discovered counterfeit coin is a class A copy as described on the value of a counterfeit 1804 dollar faq page then there is some value but for many other. If you have a magnet that is strong enough, you can pick up even weakly paramagnetic materials. But not all counterfeit coins get attached to a magnet as there are many counterfeit coins made from copper with a thin silver layer pretending to be a silver coin.