The official site relies on Disney uploading updated menus (doesn't happen all the time). The other two primary sites rely on guests reporting updates to them.
What I do is take a look at all 3. Look for the one with the latest date and estimate based off of that menu (whichever it may be). Know that things can (and often will) change between now and your trip as well. (Stupid Artist Point taking Venison Pot Stickers off the menu just weeks before I showed up

).
For the official Disney menus, go to the Walt Disney World site and click on "Where To Eat", this will take you to the base Dining (which happens to also be the primary Reservation) page. Along the right, about halfway down there are options to view info about restaurants (Restaurant List), go ahead and select something in there and click Go. You will be shown a list of restaurants based on your selections, but more importantly, there will be a set of filters along the left side. Utilize these to navigate to the restaurant in question. Select the restaurant and the menu options will be visible.
Now, how do you know the date on the WDW official menus? It's relatively easy. Along the bottom there should be some sort of code. Somewhere in that code is a 4 digit number, this number represents the date that menu was released. For example, the
official Liberty Tree Tavern Lunch menu has a code of "MK3404 9288000 0810" along the bottom left of the menu page. The 0810 tells us that it was released in Aug 2010. I don't really know what the other numbers mean, but probably stuff that we don't need anyway (well, I think MK stands for Magic Kingdom, I'm not positive on that though

)