First off, separate the jewelry into two piles, preferably on a soft surface. Be careful untangling necklaces, etc, take your time. If you have any pearl necklace strands, check to see if they are knotted. This is an indication of quality. Put to one side the jewelry you think looks expensive, the ones you think might contain diamonds etc. the "fine jewelry". Be extra gentle with the pearls please.
In the other pile place the jewelry you consider might be costume jewelry or looks less expensive. Keep in mind, that some costume jewelry is designer and is collectible and hence of value.
Now that you have two piles of jewelry in front of you, starting with one item of jewelry at a time, examine the jewelry all over, and I mean everywhere, reverse of item, the pin stems, the ring bands, the necklace clasps etc. with a jeweler's loupe or magnifying glass to check for the following:
1. Jewelry marks and letters, signatures, numbers, etc 2. Jewelry Repairs.