If you have a reasonable amount of jewelry to sell, I suggest investing in a good scale that weighs in troy ounces. These are available on Ebay at very good prices (less than $20).
Troy ounces are the units used in gold trading, not the usual ounces we use when buying fruits and vegetables.
Also remember that most jewelry is not pure gold. Pure gold is rated as 24 karats. Most jewelry sold in the US is 14 karat so when you weigh the piece, multiply that weight by 14/24 or 0.583. Similar factors apply if the jewelry is 18 karat (0.750) or any other value. Once you have done that, look at the latest gold price and calculate the value of your jewelry using the weights, the karat factor, and the gold price.
Now you have the current value of your jewelry. Unfortunately, all gold buying merchants will only pay a fraction of that value since they are in business to make money. You should go to at least 3 stores and have each of them give you a price that they will pay and the weight they have determined from their own scales. Then you can compare the weights and the prices.
You can sell to the highest bidder, keep looking, or wait for gold to rise in price, but at least you will be in a better position to know how much your jewelry is worth.