Most often, people use the Insight ability to decide whether a person is lying. Well, a player can use it for more than that – insight is a check of another character’s mannerisms, speech patterns, and body language.
It can also be used to help predict what a creature will do next (PHB 178). A good insight roll (and a willing DM) can inform a player of whether an opponent is going to cast a spell, run away, or strike with a melee weapon.
DM-Note: for ability checks during combat that involve what a character knows or observes (Arcana, History, Perception,Insight, etc.), I suggest allowing them to be free actions, and not have those observations consume an action or bonus action
Pro-Tip: Ever find your character without anything helpful to do this turn for some reason? Try utilizing an insight check and the Help action – if you communicate what you learn, without letting on, you can give an ally ADVantage, and tell them what will happen next.