Antique pianos are sometimes worth quite a bit of money, but for every piano that sells for a high price, there are several more that are barely worth anything. Sellers sometimes get distracted by the emotional connection they have with the piano and think that the value is higher than it actually is. Before assuming that it's worth more because your grandma or great-aunt played it, you need to determine what it's actually worth on the secondary market, to both collectors and players.
Check the piano carefully for any signs of a label or maker. Most manufacturers signed their creations and some are worth more than others. Examine the underside of the piano for any signs of a label and also the inside of the piano, especially under the lid.
Look at the condition of the antique piano with a critical eye. Even minor imperfections or flaws make the piano worth less. Some collectors are also leery about buying a piano that's missing any original parts or pieces and those with a new finish or stain on the wood.
Factor in the condition of the piano in terms of playability. An antique piano may be worth a few hundred dollars "as-is" because it needs several thousand dollars worth of work before it's ready for playing. A piano that plays perfectly is worth significantly more than one that needs work.
Find the value of the piano in "The Piano Book" by Larry Fine. The original book was published in 1986 by asking piano sellers the prices charged for different models. A supplement was added that covers prices up to 2007-2008, with different prices listed based on the condition of the piano.
Get a piano appraisal from a company such as Concert Pitch Piano. The company does appraisals based on digital pictures and sends your response in the form of an email. A comprehensive appraisal tells you details about the piano, but the basic package includes the price it's worth on the secondary market and its replacement value.