I believe tax is 6.5%.
On Disney World's official website, most of the restaurants have an updated menu and they include prices, before adding tax. These are for a la carte restaurants though, not all you can eat places or character dining. I called Disney Dining recently for prices of our character meals and they gave me the prices for my exact dates (mid-July so seasonal surcharge is in effect), including tax. They specified tax is included in the price they quoted me but not gratuity (except at CRT of course, where both tax and gratuity are included). There are, however, some restaurants whose menus are not on the official site (San Angel, Chefs de France are a couple I remember).
I look at the menus on the official WDW site like this...at the top of almost every single one (or at some point on the menu), they have the non-alcoholic specialty beverage listed with choice of either Tinkerbell or Buzz Lightyear (with some kind of lemonade I think). The ones that say Buzz Lightyear are updated. If there are any that still say Tinkerbell or Lightning McQueen, those are not as recent.