As far as I know, you can’t tell if someone is in a fleet unless you’re in the same fleet. As for a player in a fleet firing on you, it doesn’t justify flagging their entire fleet, since the fleet may disapprove of the actions of a single member.
I was recently in a mining fleet with my alliance, and a member of our alliance joined us on grid, but refused to join our fleet. The FC commented there was no way to tell he was not a spy, potentially fleeted up with enemies laying in wait to attack us.
There have been many times when our mining fleets have come under fire, but only those who engage targets (or are themselves engaged) are flagged. Those of us in slower ships who needed to dock and switch to combat ships could do so without combat log-off timers.
CONCORD recognizes the rash actions of a rogue are not inherently representative of a fleet, but I still wouldn’t want to be anywhere near a rogue when they get graped.