And how do you go about looking at one's soul????
By gazing into his eyes. "The eyes are the windows to the soul."
Psychologie is looking for the soul psyche-נשמה
What is a soul?
Uhhh, good luck buddy. Last I checked, souls didn't have a visible or physical form.
It's the invisible part of you that looks out through your eyes, thinks, feels, makes decisions, remembers - lives and departs from your body when you die; it's what makes you YOU!
Why על הנשמה and not בנשמה ?
Well, הִסְתַּכֵּל to look at is usually followed by עַל in Hebrew. Technically בַּנְּשָׁמָה would be into his soul, which is the more usual expression in Hebrew too, if you gain insight into someones inner life.