From overhead can you see a slight indentation of a waist and can you just feel her ribs under a thin layer of fat? That's usually the test for a dog or cat to determine whether it's under or overweight. It's actually better for the pet for health and longevity to have a pet slightly underweight than overweight, unless it is being bred.
"We humans are indeed fortunate if we happen to be chosen to be owned by a cat." -- Anonymous