How can I tell if something is real gold?

My little sister found some Captain Morgan Spiced Rum coins in my great grandmothers old apartment. At first we thought it was plastic, but it was cold to the touch. On the back, instead of saying “No cash value” it says “one piece of eight.” It isn’t attracted to magnets, it is scratched by a coin and it has the same color as gold, but how else would I be able to tell? I imagine the coin is mixed with another metal because, where there aren’t any words or symbols, it is quite smooth.