When a dog has ear mites they'll usually shake there head excessively and scratch their ears a lot.
Dark, crumbly reddish-brown discharge from the ears is also another indicator that your dog has ear mites.
Sores on the ears are also another indicator of ear mites and even seeing ear mites on the ear itself.
You can treat the ear mites and ear mite bites with some over the counter ear mite drops that you apply to the dogs ears.
Clean your dogs ears first and then put the ear mite drops in the dogs ears and massage the ears.