Some monsters seem to attack on sight while others of the same types are happy to leave me alone. How can I tell what monsters are which?
Additionally to Indigens that attack on sight, there are also those who attack when you get too close; those have a lightning bolt symbol instead of an eye one. Many Indigens have both symbols.
To follow on from Nolonar the lightning symbol actually means "auditory type" where they will attack on sound instead of sight. Chapter 11 page 2 of the in game manual.
Some monsters have a red eye shape next to their level. This means that if you are in their line of sight they will attack and you will "Flinch."