Since no one else has chimed in, I’ll take a crack at it, although I have no particularly brilliant ideas on the subject. My elbow position is a result of other forces, rather than a decision to put it high or low. I try to pull straight back on the string, which puts my elbow more or less in line with the arrow. I can feel additional pressure on the top of my index finger if my elbow is too low, and additional pressure on the bottom of my ring finger if my elbow is too high. I couldn’t really tell you exactly where my elbow is, but I can feel the pressure on my string fingers equalize as I move my elbow into the correct position as I draw the bow.
I shoot three under, but I suppose another indicator if I shot split fingers would be excessive pressure on the bottom of my index finger if my elbow were too high.