You need to be very conscious before buying anything in gold. First of all there should be a label on the gold item. Normally in rings, it comes inside the ring and on stuff such as bracelets and necklaces, it is on the clasp. The label must say 10k, 18k, 14k or 22k which shows that it is real karat gold.
You can also check out for real gold through a test. Test the density of gold by using a graduated cylinder and a scale. Weigh the gold and put some water in graduated cylinder and note the level of water. Then put the gold inside the water and note the water level again. The difference between the two measurements will give the volume of gold. Density is equal to mass/volume. So divide the weight of the gold with the volume. The answer should be around 19 gms/cm cube for real gold.
You can also use vinegar to check for real gold. Just put the piece of gold in vinegar or nitric acid. If it is real gold, it will not change and fake gold will change.