All that glitters is not gold, though telling the difference can be tricky. What may look like a simple gold chain may actually be brass, an alloy of 67 per cent copper and 33 per cent zinc. Both are lovely to look at, but there are several easy tricks to tell the difference.
The simplest (though admittedly not entirely scientific) way to tell whether a chain is gold or brass is the smell test. Rub the chain between your fingers for several moments, long enough for the chain to warm slightly. Set the chain down, then smell your fingertips. If there is a slightly metallic odour, your chain is not gold. If there is no odour, you chain is either gold or covered by an extremely thick gold-plating. While this can tell whether your chain is gold, it cannot necessarily confirm whether your chain is brass.
Conduct a density test. Gold is significantly more dense, about 19.3g/cm as opposed to 8.5g/cm for brass. A simple way to test the metal's density is to fill a cup to brimming with water, then place the chain in the container. Catch whatever water spills over and weigh the spilt water. Remove the chain; weigh it separately. Divide the weight of the chain by the weight of the spilt water. If the chain is gold, it will weigh 19.3 times more than the water; if the chain is brass, it will weigh about 8.5 times the water.
Apply a small dab of muriatic to a link in the chain. If the chain is gold, the metal will remain unchanged; if it is brass, the chain will change colour or disintegrate. Use this method only if you do not care what happens to the chain after your test, because the results are irreversible.
Visit an experienced jeweller to find out whether your chain is brass or gold. He will have the know-how to definitively conclude whether your chain is gold or simply a very charming approximation.
Exercise extreme caution when using acid to test metals, because these chemicals can be extremely harmful to one's health. If ingested, call Poison Control immediately.