Gizmo gets a health check every morning, and I highly recommend it for all pet owners. It helps you to recognize what's normal for your pet, and lets you catch problems really early on. And, as an added bonus, it gets them used to being handled.
I feel all along his back and sides for any lumps or bumps, his feet and pawpads get looked at to make sure they're in good shape, and I look in his eyes, ears, and mouth. I also check under his tail just to make sure he's keeping the fur there clean and dry.
Important Note: Always end the health-check with ear-rubbings. Otherwise, the fox will be obligated to scream at the top of his lungs to let the entire county know that you're skipping the most important step, and that you're neglecting him.