Yes, you have to use the entire CS or TS credit at one time. There is no way to "save" part of a credit to use later on. If you choose not to order a dessert or beverage or an app (on the DxDDP) at the time you use the credit, you will lose that part of the credit.
At some of the food courts in the resorts that have ice cream as a dessert option, you have to tell the cashier you want ice cream when you purchase your entree and beverage. Your receipt will be marked with a time limit (about an hour) so that you can come back for the ice cream when you're ready for it. This way you won't have a melted dessert, but there's no way you can go back several hours later or go elsewhere to get your dessert.
As far as knowing how many credits you have left, every food receipt will print the number of remaining TS or CS or snack credits. It's always a good idea to check when you get the receipt to make certain it is correct.