Is he REALLY giving you any sign or are you not catching it?
What I did with Matty, when I think it's about time to go, I took him to the backyard door, ask him to sit, and then let him out and then keep saying "Pee Pee". When he pees I praise him.
So in a couple days he learned that when he needs to go, he goes over to the backyard door and just sits there.
The only tricky part is when we're upstairs he didn't know how to tells us he needs to go (no backyard door upstairs! And he couldn't quite figure out the stairs connect upstairs and downstairs at the time) so a lot of times accidents happened upstaris. Nowadays he'd just come over and sits in front of us if we're upstairs and he needs to go. We didn't know that was the sign initially ... took us a while to figure out. Initially he'd come over, sat in front of us, and we'd praise him "good dog good sit" ...
SD