The brochure that comes with the plan will help a lot.
The general rule of thumb, though, is that at table service, it's either a buffet/family style meal or you are seated by waitstaff and your order is taken from a menu. Counter service are locations where you place (and pay for) your order at a register or kiosk and typically are served right there, allowing you to take your food to an open table of your choice (there are some exceptions here - places that will bring your order to your table, but you still order/pay up front).
Snacks are sold at counter service locations and at a variety of other spots that don't serve full meals. They're in some ways the easiest - a purple
DDP logo on the menu indicates what's eligible as a snack item.
I should note, if you're going in May and if you're having confusion over what a table service location is, you may run into trouble securing reservations, which are
strongly recommended for all table service locations. One of the drawbacks to using the dining plan is that you really do need to plan in advance what table service restaurants you are going to eat at - leaving it to trying to secure walk-ups or day-of reservations is... risky at best.
