No matter what the counterfeit, there's always a way of testing, and due caveat emptor and diligence applies for any buyer. And as tricky as it might seem to pull it off with the same weight, dimension, etc., it is being done.
As you can see from the video, Silver Eagles are routinely counterfeited, even though the original counterfeiters are "above board" about it (with the usual sleeze words -- e.g., "100% pure silver clad over 1 oz.!" -- words that imply to the truly ignorant that they're buying basically the same thing). My brother has two China-made Silver Eagles that he bought off the street in Shanghai for 50 RMB each (about $7.50). They are silver clad, but because of the based metals used, they weigh exactly 1 oz., the same as a Silver Eagle. The only surefire way to know the difference, as you mentioned, is to file into the side grooves and do an actual acid test. He doesn't have to do that with his, because he knows they're fake.
The point was not that counterfeits couldn't be tested for and detected, but that very good ones do indeed exist, which can be a problem for anything as popular as the Silver Eagle. That same problem does not exist (of which I'm aware) for old circulated lower denomination junk coins. US minted coins that are passed off as proofs, uncirculated, or of extra fine condition are another story. Again, however, that's for larger denomination uncirculated coins and proofs, like Morgan and Liberty dollars.
