When I first adopted Snoop, I had to decide how and where I would train him to use the bathroom. I chose to let him go potty outside whenever he needs.
The easiest way for him to let me know he needs to go: he just rings a bell!
When he was just a puppy I tied a rope string around a large jingle bell that's glued to a cork. I either tied the string around the door knob/handle or just tied in on a hook next to the door.
Before each bathroom trip, I would ring the bell so that he associated the sound of the bell with going outside to potty.
By practicing this simple step, I had Snoop bell trained within a just few months. Now when we he needs to go, all he has to do is ring the bell and I am trained to open the door.