Get multiple opinions before selling or trusting a single number. Start with a professional coin grading service like PCGS or NGC for authentication and grading; they’re neutral third parties with no stake in buying your coins.
For a value estimate, visit two or three local coin dealers separately and compare their offers, since prices can vary widely. Avoid appraisers who also want to buy the coin on the spot, since that’s a conflict of interest. Check recent auction results on sites like Heritage Auctions or eBay’s sold listings for similar coins in similar condition.
Join a local coin club (ANA affiliated clubs are a good bet) where members can offer honest, informal opinions. Never let anyone rush your decision.