[Baertram]

There is nothing API like that will tell you you are in a red circle. You simply need to use the game's idea: Chaneg the color to somehting more visible in the settings and use your eyes, or learn what bosses and enemies say to the NPC chat before a red circle appears (mechanics).
You can only react on incoming damage, and then it's too late ...

That said most circles and ingame 3d stuff is not accessible via addons or the API. You can add your own textures and markers but what isn't given is information about enemy/player positions movement (except for grouped players -> position).
If there are buffs on your player or the enemy you are able to read you can react on this. But animations of weapons, enemies and so on do not trigger any event or api stuff you would be able to react to. Sometimes there is a chat NPC message, sometimes there isn't.

There are some major warnings if you enable them in the settings like "dodge" or "block" which you are able to use within addons.