You can perform some tests yourself on the gold.
Looking at the gold, are there any black, silver or other color blotches in it?
When you hold the gold item, does it feel heavy?
Does it have a karat stamp on it?
The following table shows common locations for the karatage stamp.
Jewelry Type:Stamp Location
Ring: On the inside surface of the ring.
Earring:On the post.
Pendant:On the back. Sometimes on the chain loop
Bracelet:On the clasp.
Necklace:On the clasp.
Gold is the only yellow metal that does not react with nitric acid. Test kits using an acid are available at jewelry supply stores. The test usually involves rubbing a miniscule amount of the metal on the supplied paper and putting a drop of the acid on the paper rubbings. If the color does not change, it is real gold. 10K gold or less may react to this test due to the low gold content.
Another test you can use is the price test. If the price is too good to be true, it probably is. To figure out the value of the jewelry, you need to know the current market value for gold, the weight of the gold jewelry in ounces and its karatage. The value can be calculated as:
value = (gold market value) x (weight in ounces) x (karatage/24)
For example, if the price of gold is $900 per ounce, and you are looking at a 22K gold pendant that weighs 1/4 ounces, the formula would be:
Value=$900.00 x (1/4) x (22/24) = ?
Value=$900.00 x (25) x (91.667) = $206.25
The price can be higher for workmanship or branding, but if the asking price is $100, you can be sure that the pendant is not 22K gold or even real gold.