I usually feel along the spine and ribs and at the top points of the hips. You should be able to see the waist and feel her ribs under the thin layer of fat, as catlover2 mentions, but you should also be able to easily discern the tips of the hip bones by touch. You don't want her boney, but the bones should be easily palpable. Every dog is different, though. We have one who looks ribby even if she's got excess fat along the spine and over the hips.
If you have any doubts about it, a groomer or a vet should be able to tell you if your dog is at a good weight. Once you've determined what a good weight 'feels' like to your touch, you can check her once every few weeks or every month and keep her on track