Use a strong magnet over the item. If it attracts the magnet, then it may contain iron or steel. However, the magnet does not detect tungsten which a high density metal that is as heavy as gold.
Hold the gold chain or coin in your hand to do a quick assessment of its weight. Real gold is very heavy since its density is 19 times heavier than water. It does require some experience to distinguish the densities of different types of metals. Gold is approximately twice as heavy as silver, copper, and palladium.
Inspect the color of the item in question. Real gold is deep in color, not bright yellow. a quick comparison would be matching it against copper, bronze, and other lemon-yellow items. Depending on the carat, its color would slightly deepen accordingly.
Scratch it against a precious metal testing stone. Real gold should scratch very easily. Steel or Palladium would require much pressure and friction to produce the gold mark on the stone. Then, drop a testing acid against the residue. Depending on the carat, observe whether the acid is able to dissolve the scratched gold residue.
There’s a saying that “true gold is never affected by fire.” Therefore, a propane flame (or acetylene/oxygen flame) should be used to test deep within the gold. The color of the gold should not be any different inside. Also, if this item is palladium it would require about 40% more heat to melt.
An electronics gold tester, such as the Mizar 24K or Tri Electronics Pro 24K, can be used to test the actual carat.
The cleanest way to evaluate gold is to use a graduated cylinder to check the mass of your item. Then use a precise scale to check its weight. Divide Weight by Mass would give you density. 24K gold would be about 19 grams per mL. Any result less than 14 grams per mL would be considered fake gold.
Chemical tests are imperfect due to possible sampling errors. The good thing about 24K gold (Au) is that you can tell by the size (density), soft, a very specific light tint gold color, and resonant frequency of 1.754MHz. Au is 19.3 g/cm3 – that’s way more dense than lead 11.4 g/cm3 as a comparison. Tungsten is the other known element that can closely match Au in terms of density, but it has the physical properties of being ‘brittle’ (not soft) and a resonant frequency of 7.83 Hz (almost same as rocks). Therefore, one can choose to drop a coin or bar onto concrete and listen to the resulting sound. If the gold coin is impure, it will produce a thumping sound, rather than a high-pitch resonant.