Look for changes like loss of appetite, lack of activity, sensitivity to touch, sitting on top of his paws, or acting withdrawn and hiding. (pets.webmd.com)
For cats, you may dab gel on top of their paw as they will lick it off as well. (petzlife.com)
His skin is black and leathery, and he itches a lot, and chews on top of his paws. (cesarsway.com)