A quick do-it-yourself test is to rub a corner of the object between your fingers, and smell your fingers. If you smell anything...it's not gold.
(Otherwise, you can do a weight / density test. Just weigh the water the object displaces against the weight of the actual object. I can't remember exactly, but gold will weigh something like 19x the weight of the water, while brass will be be like 8.5x.)