The only things that Disney will guarantee as a "snack" are the items listed on the official brochure, which are items available just about everywhere: ice cream novelty (mickey ear bars, etc), fruit, bottled soda and water, fountain drink, coffee, bag of chips.
Anything else that is available as a Snack is set by the individual location. As mentioned in previous posts, those are *generally* single-serving snack-like items under $5 (though most are under $4). But that's far from an exact criteria to use. At one time Flame Tree BBQ offered a basket of onion rings for around $4 that you could get with a Snack, but a cheaper basket of french fries you couldn't.
The main thing to do, also previously mentioned, is to look for the purple-and-white DDP logo on the menus next to items that can be purchased with a Snack credit.
And one side note in terms of using the credits, if you're using both a Snack credit and a Quick Service meal credit at the same time, they'll have to be rung up by the CM separately. It's a quirk/requirement of the cash register system that only one type of DDP credit be used in each transaction.
-Rob