gold may get dirt on it but it doesnt tarnish . Copper tarnishes. A "touchstone" is a piece of unglazed procelain where you rub the coin edge. If its gold itll show yellowish, if its copper it comes off much darker . Copper scratches gold but gold wont scratch copper.
Id say rub it on a shallow nappe carpet along the edges and see if they brighten up, then itd most likely be a copper coin. The difference between copper and gold is really quite different and you should be able to tell at a glance