How do i train my dog to let me know when he wants to go and pee? help please?

I have a 19 months male boxer he has been died, i have a huge problem he doest let you know when he wants to go to do his buisness, he never barks( which is not a bad thing) but he just goes to the door and if your distrated or in another room and dont see him he will just pee. i want to teach him to bark or something to let me know he needs to go out? help please.