What you should do is count from the day she returned forward 21 days.
You should notice that her nipples will be a bit puffy, and pinker than usual. At 21 days, that the only way to tell... I dont think even a vet will be able to "feel" kittens yet. If you determine that she is pregnant, then she should be started on kitten food to boost her nutritional requirements. Make darn sure she doesn't get out again! If she isn't pregnant, get her spayed, that way if she does sneek out, you'll not have to worry.