In 1925, the United States did not issue Walking Liberty silver half-dollars, which were the standard circulating halves at the time. But the U.S. Mint issued four different commemorative silver half-dollars -- special strikes honoring various events -- that bear that date. Their value depends on collector demand for the coins, which is largely a function of their condition.
Determine which 1925 commemorative half-dollars you possess. The four coins issued that year are the California Diamond Jubilee Half-Dollar, the Lexington-Concord Sesquicentennial Half-Dollar, the Stone Mountain Memorial Half-Dollar and the Fort Vancouver Centennial Half-Dollar. All are dated 1925, but if you're not sure what you have, images of these coins are available online.
Estimate the condition of the coin. Good, Very Good, Fine, Very Fine, Extra Fine, Almost Uncirculated and Uncirculated are commonly used grading terms. On one end of the scale, Good coins are heavily worn. On the other end, Uncirculated coins show no evidence of wear on any surface and have a shiny luster. Be advised, however, that coin grading is more art than science, and opinions will differ about a coin's condition even among professional dealers and collectors. Still, many 1925 halves will be Uncirculated or close to it, because they were created as keepsakes.
Check standard published references, especially the latest edition of "A Guide Book of United States Coins" (Red Book) and "The Handbook of United States Coins" (Blue Book) for "retail" and "wholesale" price estimates for your coin. Retail is what you might pay a dealer for a coin, while wholesale is the smaller amount that a dealer might pay you for the coin. Also, look online for sites that offer coins for sale, such as specialty coin dealers or general auction sites, for a sense of how much it might fetch.
Take your coin to a dealer you trust for an estimate if you are seriously interested in selling it. An honest coin dealer will tell you, based on the coin's grade and his knowledge of the coin market, how much you might be able to sell it for. The dealer might also offer to buy it, but you are under no obligation to sell it.
Have the coin graded by either of the two main grading services, if you want to sell it and you believe your coin has more than ordinary value -- more than $100 or so, as a few of the 1925 commemoratives trade for. That will eliminate some of the uncertainty about its grade and thus make it easier to sell. The two services are the Professional Coin Grading Service and Numismatic Guaranty Corp., each of which charges a fee for the grading service.