Any time you are in doubt, ask the counter person or the check-out person.
In general, if the item costs less than $4.00 for a single serving it is allowed as a snack on the dining plan. So for examply, a piece of cake listed at $3.49 would count as a snack at a counter service restaurant (or the dessert on your counter service meal.)
For counter service meals, if there's a "combo" platter or meal offered, it will count as a counter service meal. Example: hamburger & fries, soda and dessert is one counter service meal. But there are many choices in addition to burgers & fries.