When buying gold jewellery, look at the BIS logo, purity of gold, and HUID number engraved on it. The hallmark provides information like the content and manufacturer of the item. 

Take a few drops of vinegar and put it on the gold, if it changes colour then it is fake, if not then it's real. 

If a big piece of gold is too light compared to the real gold, it is fake. Checking size and weight is a standard procedure to check the authenticity of gold. 

Rub the gold on an unglazed ceramic tile or plate, if it leaves a black mark then it is a different metal or gold-plated. 

Gold does not react to a magnet thus it won’t affect it. But if you see an attraction then it is fake. But it's not 100% reliable as gold-plated metal also doesn't react. 

Take a cup of water and drop the gold in it, if it sinks it is real as gold is a dense metal. It floats or stays in between, it's fake.