How does one come to know of this 'sa'? Naturally, from listening to the great masters. It does not matter if you have sur gyan or not. When I listen to Malikarjun Mansur sing, I can tell I am listening to the pure 'sa', the precise 'sa', exact 'sa'. And, of course, the pure re ga ma pa dha ni too. I can feel the difference even though I am not versed in sur gyan, or in the shashtriya sangeet pranali. Just like I can tell the tuneful from the tuneless I can tell the pure 'sa' of a master from the 'sa' of those who just sing tunefully. The notes have a bell like clarity that penetrates to the core of one's being, making it quiver. And I can tell when the roti I am eating is cooked just right. It melts in the mouth and tastes like heaven.
Getting it just right is a matter of practice, or, as a musical master would say, riaz. One practices endlessly for years, same thing day in and day out, whether it is the sargam or it is baking chapaatis, till one day the note flows effortlessly from the throat, and the chapaati comes off the tava - seemingly of its own - the moment it is cooked just right. Unfortunately, the importance of precision, or riaz, is not taught in the schools and universities of the modern world, except as part of some notion of mathematical or engineering exactitude, necessary to make things fit well, thereby minimising friction. But precision is not just a skill. It is an art, a philosophy, a way of life. It is an essential ingredient of both harmony and happiness.