1. Check if there is there an official marking. 
2. Check for discolouration.
3. Carefully bite your gold (do not damage your teeth!). 
4. Use a strong magnet.
5. Know that the higher the density of the metal, the more chance that it is gold, and the greater the density of gold, the purer it is.
6. Test against fool’s gold (beware as your piece may get scratched). 

How to Tell if Gold is Real

Knowing how to tell if gold is real can save you lots of your hard earned cash, or, conversely, make you a fortune from something that you originally thought was worthless. In general, anything under 10 karats fits into the “worthless” bracket. Should you not be able to find a certified jeweller to have the gold tested properly, here’s how to do it yourself.

Steps

1- Check if there is there an official marking. There may be a hallmark, a mark detailing how many karats it is, or a measure of fineness (.1-.999). Just because it doesn’t have a hallmark or similar, it doesn’t mean that it’s not real. Old gold may not be marked or the mark may have worn off. Be aware however, fake pieces may have a mark that appears authentic.

2- Check for discolouration. If the gold is wearing off to reveal a different colour metal underneath, then it probably isn’t real. 

3- Carefully bite your gold (do not damage your teeth!). The purer the gold, the deeper the indents from your teeth.

4- Use a strong magnet. If the metal is attracted, then it is not gold (but just because it isn’t attracted it doesn’t mean that it is gold!)

5- Know that the higher the density of the metal, the more chance that it is gold, and the greater the density of gold, the purer it is. To test this, weigh the gold in grams, then place it in a finely incremented measuring cylinder that already has a measured volume of water in it, and then measure the new volume. This will give you its volume (measure in ml). The density of pure 24K gold is about 19.3 g/ml. Density = mass/volume. 19g/ml indicates real gold or one of the few metals as dense as gold, however density can drop as low as 12.9 g/ml for 14k gold. Different gold purities will have a different g/ml ratio:

6- Test against fool’s gold (beware as your piece may get scratched). Find an unglazed ceramic plate and run your item across it. If it leaves a black streak, it’s not real, but if it leaves a gold streak, it is real. 

Tips

Jewellers may use nitric acid to test gold. Due to the difficulty in acquiring this substance and the danger of keeping it, it is best to leave this test to the professionals.

Not one test is fool proof. Instead, use tests in unison to get a more confident feel for the metal.