I chart my moods. Stable is when I have a straight line indicating I am neither too high nor too low with little fluctuation. That way I know my mood stabilizer is working. I still get depressed but it comes on very gradually and by charting my moods I can see it. It is my evidence. Then I can take steps to fix it. I don't focus on a single day but look for patterns. Every so often I will have a bad day and that shows up as a little blip. I try to normalize that by saying to myself that people without bipolar have bad days too. Again, I look at the big picture to define my stability.