Sleeping more than usual is a big sign, especially in kittens. Lack of interest in things that they would normally be interested in - playing and eating especially. If they are feeling ill they may sit in a hunched up position, crouched with all four feet under them rather than lounging about the way they usually do.
Cats can and do vomit occasionally due to perfectly normal reasons such as to get rid of a furball or if they have eaten too fast.
If she's just been sick the once, and is otherwise acting completely normal, then don't panic! Keep an eye on her, if she vomits (or has vomited) several times or becomes lethargic as I describe above, then she needs vet care (phone them up and ask about it if you are still concerned), but vomiting just the once is not necessarily anything to worry about