It is immensely difficult.
This was fictionally explored in a Tom Cruise movie where future crime was detected with a device and psychics to arrest people before the crime was commited.

You can't arrest everyone who acts wierd but if the individual has a cache of weapons what is the harm in taking a second look?

One harm was the seige at Waco.
Ruby Ridge
The fire bombing by Philly police that killed a black community
SWAT team murders resulting from a wrong address
etc etc

under react vs over react

its a tough nut

One thing I could say is if you live by the gun you might die by the gun.