You can identify your Depression Glass by checking out marks on the bottom, but you also need to know the pattern. If the mark indicates a particular company, that is a help, but most companies made several patterns. You can identify the pattern by using one of the many books on the topic. Books show you how to identify patterns by studying shape, edges, color, size, and design. Designs can be floral, geometric, can show scrolls, lattice, or other elements. If you look at the products of a particular company, you will find out that certain patterns made by that company are in higher demand than other patterns made by the same manufacturer.
Once you learn more about each piece, you can search online to see what each item sells for. When possible, check the sold price. Sellers may over value their wares, so just because you see something offered for sale at a high price does not mean the seller will actually achieve that price.