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.