If you are on the Disney Dining Plan, when you check in to your WDW resort, you will be given a brochure about the Plan, which includes a listing of all the Disney restaurants that participate.
If you'd like to get a look at this in advance, the Disney Dining Plan brochure can be downloaded from the WDW website.
This includes a listing of all restaurants that are on the Dining Plan,
as well as additional information about number of credits used, counter
service vs. table service, etc. To download the brochure, click
HERE, then click on the Disney Dining Plan card, then click on "Download Disney Dining Plan PDF."
To figure out which items count as snacks, you'll look for the little "snack" symbol on the posted menus at the counter service restaurants, on the food & beverage carts, and at the merchandise locations. The symbol is a small purple and white checkerboard. As soon as you know to look for it, you'll see it everywhere.
Enjoy your visit to Walt Disney World.