Yesterday, 02:36 PM
Buying from a large, reputable dealer will reduce this risk tremendously.
There are conductivity-based electronic testers. They are what pawn shops seem to generally use. I would not recommend you buy gold from pawn shops, though. If you are an expert like oyarde, OK, but if looking to avoid risk of counterfeit, stick with only the best, established, reputable gold dealers.
So, a "ping" sound drop test, an electronic (conductivity) test, a visual examination, a density test, all these are measures to take to try to determine the authenticity of the gold content. The acid test oyarde mentioned is another possibility. If really serious, the X-Ray fluorescence test in dannno's link would likely provide the best non-destructive/invasive (no melting or boring) test of the coin. You could perhaps ask around at a local university to see if they'd let you use one.