Usually when your dog has ear mites you'll notice the dog scratching it's ear and shaking it's head more often.
If the dog has ear mites you should notice some sores and even the ear mites in the dogs ears as well.
You can get some drops to apply in the dogs ear and on the dogs ear to help kill the dogs ear mites and it's best to apply some ear mite drops as well after getting rid of the ear mites to keep the ear mites away.