How can a "layperson" help figure out what body parts are "causing them grief" doing a technique, exercise or particular drill...?

The body is a fairly complex organism with all kinds of interconnected (or is it inter-related ?) parts, such that when you put them together, and then move them in different ways you have all kinds of potential sources to isolate & explore.

If the X-rays come back "fine" and the "white coats" (ie medical doctors) say there's no anatomical issues, impeding somebody... how the blazes does someone identify the muscle, muscles, tendons, ligaments, etc.... in order to make positive changes, no matter how small???

For the sake of discussion, lets try... I don't know, sitting on the floor...soles of the feet pressing together in what many call a "butterfly" stretch.

How would you ever figure out what keeps one leg-knee FAR higher than the other while attempting to press both knees toward the floor? How would you go about figuring out what the offending muscles, or groups of them are ~interfearing~ with the simple lowering of ONE leg (even close to horizontal) to the height of the opposite leg ?

Thoughts Anybody?
Jeff