The magnet test: if there's iron in it, a magnet will attract it.
The acid test: pure gold does not react with nitric acid.
The density test: pure gold has a density of 19 g/ml which is higher than most metals. 18K gold has a density of 15 to 17 g/ml.
The wear test: if it's just gold plated, the plating may flake off revealing a different color.