[header] How to teach a dog to tell you when he wants to go outside [title] Hang a bell by the door. [step] Make sure the bell is within the dog's reach and is loud enough that you will hear it even if you are not in the same room. It's also important that it be durable enough that your dog won't break it. 
[substeps] You can also use a wireless doorbell, as long as your dog will be able to push to button. If your dogs seems afraid of the sound of the bell, try dampening it with some tape to make it quieter.