Our 2 yr old is completely house trained (I could only dream of having him potty trained

) in our house.
For the most part, the other houses he would be in are homes with other dogs and one has several including puppies 2-3 times a year so there is almost always a "sign" of other dogs going in the house. When there, we watch him like a hawk and he'll usually walk up and we can tell he's about to hike and we'll just tell him "no" or if close enough, push his leg down. But if we don't, there's a chance he'll hike to mark. He's still unaltered at this point (another week or so)
The only other house he'll be at once in a while has no pets and he won't go anywhere but outside. But we still keep an eye on him and take or let him out often enough. We'll also make sure to walk him when there as that "always" gets everything moving quickly.
We have people that swear we're just not finding where he's marking in our house because he's unaltered, but he just doesn't. Persistent positive reinforcement training works! Same thing has work with our female now. At 9 months, in the last 3 months she's probably gone in the house 3-4 times but each time, when we see it, she tucks her ears and knows she shouldn't have. So off we go, outside to show her were it belongs.
People had told us to be sure to train them to go inside as well, on pads or paper, for bad weather etc. but I have to say, I think you have to draw the line. They either go outside or inside. We choose outside. However, for bad weather, I'll take them out on a porch or patio and allow them to go where ever they need to go. Its the only time they'll do it. They'll even go on the swim platform on our boat where an easy flush is right there.

But we stick to consistent locations.
All ad though, they now have a doggy door in the wall going outside so they cna go whenever they need to. And the male is really good about going, and she'll follow along.
For training overall, they've made me realize the only reason they dont' or can't do anything we say is because we're screwing them up!

They are smart as can be and if we are consistent, they'll pick something up within 2-3 tries. Of course that goes for bad habits as well!!