The taxis in Prague are notorious for overcharging and dishonesty. In order to avoid getting ripped off, you should always take an official Prague Taxi, which has fixed prices for specific destinations. The main way to tell if a taxi is legitimate and registered is if it has a permanently installed yellow roof lamp, the words TAXI in black on both sides, and the driver's name and rates are printed on both front doors. In addition, drivers should always offer a printed receipt. Your safest solution is to just call a reliable taxi service.