What is the ruling on looking at those who commit sins, such as one who lets his garment hang below the ankles (al-Musbil) or one who wears tight clothing, knowing that this looking is not an acknowledgment [of their sin] and occurs when seeing them on the road or when visiting relatives?

It is forbidden to look at someone committing a haram act. However, in matters of scholarly dispute, one should not condemn the doer unless one believes it to be forbidden. It is recommended to guide them if they are ignorant. The issue of isbal (letting one's garment hang below the ankles) without arrogance is a matter of scholarly disagreement, so one should not condemn someone who practices isbal. As for a man wearing tight clothing that outlines his awrah (private parts), it is makrooh tanzeehi (disliked but not strictly forbidden). It is disliked to look at his awrah as outlined through his clothes. However, tight clothing not outlining the awrah is permissible.