I would like to know this too because right now my dog just gets up and walks around and that can mean that she wants attention OR that she needs to go out and I never know which it is! And sometimes she's sneaky and just pees when I'm not looking... But she's five years. Usually pretty good but I'd like to be even more accident free!
That's how Cooper acts as well. I usually don't take a chance and just go on and take him out. But some times I'm doing something and don't see him do it. Last nite he my wife said he went to the door just before he peed on the floor and we were both preoccupied until she spotted the mess. We immediately took him outside.
A 1.5 yo Bichon is an intelligent dog. He should already look at you, maybe look at the door, sniff, rotate a little, then squat. You can learn to recognize his specific pattern. Say 'Potty' when you even think he has to go, and take him outside immediately. Continue doing this and he will learn the pattern.
Some people go a a step further. They hang Xmas bells on the back door, then when they take the dog out, they say 'potty, ' ring the bells, then take the dog outside.
I don't use that method, b/c my dog looks at me and whines when he wants to go out.
We taught our pup to ring a bell by teaching the "touch" command and then having him touch the bell when we were going out. He has taught himself the alternate method of going to the back door (now that he knows how to walk down the deck stairs!) and banging a cat scratcher that hangs from the doorknob until someone comes and lets him out. (We always go out with him)
We also say "Do you want to go OUT?" and when he does actually need to go out, he gets all hyped up and runs to the door or his baby gate. When he doesn't need to he just looks at you like you're crazy. He'll be 5 m/o next week.
At 15 weeks, I wouldn't expect him to let you know consistently. The reason is, puppies don't have full control of their bladders til 6 months of age, give or take; every puppy is different. So, there may be times when he isn't really fully aware that he HAS to go until it's too late.
A better plan would be to set a schedule and take him out, say every 2-3 hours. Gradually you can try to extend that, when the pup seems ready. And, over time, you could teach the bell thing, if you wanted to.
My adult dogs don't make any sounds to let us know. Abby stands in the middle of the kitchen and looks back at us, and Harper will come stand in front of us....