Arthritis in a dog's hips.?

Does anyone know (I know there aren't any Vet's here), how to tell the very First signs of arthritis in a dog's hips? The Vet said the dog does not have arthritis, but I disagree, (no X-rays were taken, just manipulation of the legs), as I watch how she walks and gets up daily, compared to when she was younger. The Vet says it is just because she is 12 yrs. old, and stiff. Does anyone know, in any way, if she is 'stiff' or 'arthritic'? One day after a bit of a long ride where she was laying on the back seat in the Van, she got up to go outside, and wouldn't put one foot on the ground. Just her toes. After walking her slowly, a bit, she still didn't want to put full pressure on one hind leg, just the toes, again. After a walk, if she were stiff like Vet says, I would think the stiffness would be walked out of her. Any informative opinions?