Hi Catherine!
Probably the easiest way to tell whether a restaurant is quick service or table service is to look through the window and see if guests are being served or not. Table service restaurants almost always have greeters either outside to check in guests or immediately as soon as you walk in. You'll always see the "hustle-and-bustle" of hosts/servers moving about, and in a few cases, see guests waiting outside to be checked in.
With quick service, you seat yourself, have to order at the food station (vs. being served), and generally won't find a number of guests waiting outside to be seated.
Restaurants that carry the Disney Dining Plan will generally have the Dining Plan logo either on the entrance window or on the menu screen. To my knowledge, however, there are no symbols outside the restaurant that differentiate quick from table service.
To help, our family would bring a copy of the Dining Plan to be sure.
As for snacks, Disney does have a general definition of what constitutes a snack, but you may have to ask if you have any questions. If you go to the Dining Plan (click
here to access now), there is a section on snacks and Disney does provide you with specific examples.
Hope all this helps. Have a Magical Day!