If you are just an average soldier, how do you tell if a person is worth taking hostage vs killing out right, for example a Jamie Lannister? Do High Born people wear something distinctive or do they get killed despite their potential value often/ Your real world and/or ASIOAF thoughts are welcome. Thanks!

You tell, because highborn people wear the best of everything, whether that’s armor or courtly dress, because they place their sigils/coats of arms on everything they wear, whether it’s a surcoat over plate mail or a signet ring or a cloak or a piece of jewelry, and because they’re healthy and well-fed when most people aren’t. And most of the time, that works, because most soldiers know that they could get rich by holding a highborn for ransom. 

But it doesn’t always work. Maybe the soldier in question doesn’t recognize the highborn in time because it’s foggy or the mud is covering his armor, or maybe the soldier doesn’t think he can get the prisoner off the battlefield and keep him prisoner (say if he thinks his side might lose the battle, or if it’s a quick-moving skirmish), or maybe his blood is up and the highborn looks like an enemy, or maybe he thinks it’s easier to cut the rings off the finger than risk having his captive be commandeered by a highborn officer of his own…and that’s how knights end up with misericords stuck through the gaps in their armor.