I practice Flamenco guitar. Basically all music sheets that I read indicates the left hand fingers (1, 2, 3 and 4).
This is amazingly helpful because you can complicate a lot of things if you are using unsuitable left hand fingers.

But there are a lot of music sheets that don't show the notion of the left hand, this makes me spend a lot of time trying to figure out and wondering if I'm using the best left hand fingers positioning.

How can I know if I'm laying out my left hand fingers correctly? How to develop this instinct to become natural like all guitar players?
(Just let me emphase that I read music sheets and I don't use tabs, not sure if this could affect the answers).