Vomiting and/or thin skinny stools. Either of these indicate a blockage or partial blockage. Terry cloth has an additional risk because it can have long threads which can bind the intestines separately from the cloth. (Yup, expensive "Egyptian cotton" towels with long threads are more of a risk than the Walmart special.) So that's why skinny stool are a key indicator.
DON'T give anything other than her regular food. If there is blockage and if a vet has to perform surgery, you don't want anything extra to be in there (any sort of weird substance) that will make surgery that much more difficult because the vet will have to deal with that as well.
Are there bits and pieces of the terry cloth around that suggest that she was playing with it, or shredding it before she swallowed it?
All you can really do is watch carefully for several days (24-36 hours for the big pieces and then another couple days to ensure all the strings have passed safely). But the very second that you get a feeling that something isn't right, bring your pup in. Failure to eat, or just a sense that the dog isn't feeling well, for example.
Towels are particularly dangerous to dogs. A friend actually lost her dog due to just ingesting a string from one of those expensive towels. They shouldn't be used for bedding at all. I expect your dog will be fine, and that, as Ruth says, it will all come out "in the wash."
But I keep towels FAR away from my kids.
Good luck. Keep us updated.