I pad trained Jake and Jackson since they were both babies during their first winter months, and it gets very cold here with lots of snow. But, in the warmer months, we spend lots of time playing outside. So, they are trained for both. They will let me know by coming up to me to get my attention, then they go to the door. If I am in the kitchen, they will go to the kitchen door and sit and wait for me to take them out.
I used to have a neighbor who had a small terrier mix. They had bells hanging on their door for her to ring. Alot of people find those to be really helpful.