I would say a change in crime from mostly scumbag on scumbag, to attacks on police stations, ambushes on LEO, military and government vehicles and infrastructure.
In the media, it could look like reports of 'mass shootings' but without a suicide at the end, and instead a manhunt without results.
Waco, Ruby ridges....but where the encircling forces find themselves sniped at from outside the perimeter.
Sudden retirement of police, military personnel and some politicians.
Finds of high end military hardware in raids, possibly with foreign origin.
"DC sniper" style events.
I think "motive" is where you look for organization. We have enough sheer violence right now to be a civil war....IF it was organized behind a cause. Right now it disorganized, a thousand little fights a day for a thousand reasons. If we start seeing regular violence, with a common cause behind it, that I think is where the first clues will be found that something is happening with purpose.