are you told if you have been alarted to a legion, ive never been alerted so i don't know

only when you are disabled by someone not within your battle tab range, some people alert just so their leaders can see comms on both sides.

the action of people disablising you is how you tell you were alerted.

No you are not, become you will become aware of it when many people from the same legion are artifact spamming you, disabling you, raiding you, hacking you, or leave taunting comms on your ship.

Or sometimes you may never know.

Some players also will go the easy route and flat out tell you they alerted you.