Dogs and cats both leave four toes, and only one leaves claw marks
Claws
Visible claw marks above the toes means a dog or fox. No claws means a cat. This works in almost all substrates and is the first thing to check.
Fox against dog
Fox tracks are narrower and the front two toes sit clearly ahead of the outer two. A dog's four toes sit in a broader arc.
Where to look
Mud at a gateway, soft sand by a river, and light snow. Firm ground records nothing useful whatever passed over it.
