You should be able to feel her ribs slightly, but, if you can't see them, she's probably not too bad off. Is she extremely active? Maybe get her eating a different kind of food. I forget, how old is she? Maybe even feed her less food, more times a day. Like 1/2 a cup at each meal, maybe she's just into eating smaller portions. And, you can use the guide on the bag of dog food as a guide to how much a dog of her weight should be eating. But, ideally, you should be able to see the her waist and even a rib or two when she's turning around or something. If you become more concerned, though, a visit to the vet may be the best thing for you to ease your mind.