Check the weight. Weights for coins can be found online. If the weight matches or is very slightly higher, then the coin is gold plated. If it's significantly heavier, then it's gold.
It depends on the coin. Things such as the Presidential Gold Dollar coins and Sacajawea dollar coins are made out of brass and not gold. However, historically the US has made coins out of gold...
You know when you find gold when you go to the narwhal $&@: it in the mouth then you dig down 14 feet then go bye a purple hat Then you meat a girrafigator and it pays you twenty bucks then you...