How do I know if a monster will attack me on sight?
Solution 1:
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.
Solution 2:
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.
Solution 3:
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."