I'm getting a 4 month old puppy? How did I housebreak him?

He's a soft coated wheaten terrier mix, and a rescue dog. I'm picking him up next week, but I want to know what I'm doing before I try it on him. I've grown up with dogs, surrounded by dogs, but this will be my first on my own? I get dominant behavior and praise when he does it right, but how do I get to know when he needs to go BEFORE he starts?