If you cant be sure with a visual inspection take him in. If its just a candle you should be able to see if there is redness, blistering, or any of the same things we get when we are burned. If his fur is too long to moniter the area perhaps clipping it would be a good idea if there is anything on the initial inspection to suggest more than just fur was burned. I think the easiest way to tell would (after a visual inspection for the obvious) simply be to stroke around the area and watch his reaction. If he doesnt flinch at all, lightly stroke the area directly. If still no reaction, pet as normal over the area. If the ears dont flatten, tails doesnt twitch, and there is no flinching, chances are hes ok. Burns are so much easier to diagnos that other issues due to being external, painfull, and the same in all species. If you cant see anything visually but he still flinches from petting, or if you do see burns on the skin, then you need to start considering a vet checking it out.