Background checks are as worthless as tits on a boar hog. Just because you haven't done a crime in the past doesn't mean you won't do one in the future.
Partially true at least! All the shooters who commit mass murder in America were almost exclusively peaceful gun owners before they committed thei crime with a gun.
How can you tell if a peaceful gun owner is going to become a mass murderer?
The most reliable indication could be in looking at the mannerisms being displayed by the future mass murderer.
Some traits to watch.
1. the gun owner's camo costume he wears more often than other normal clothes.
2. The gun owner's choice of human silouette targets he uses at his range.
3. The choice of a combat weapon the gun owner chooses.
4. The spoken attitude of the peaceful gun owner before he/she becomes a mass shooter.
5. The female sex can be at least placed farther down the list of future suspects.
All 5 are only clues and not positive indications of a potential mass shooter. But in instances where are 5 are relevant, the mass shooter will very often display all.
This would lead the police to at least having a clue as to who to watch carefully.
Are there other characteristics that could help the police to be proactive in forming a watch list?
There seems to be very little possibility of any gun control measures coming to America. This suggestion is at least a possible solution for eliminating some of the problem of Americans murdering each other with their guns. And it doesn't interfere in any way with the freedom to own guns of any style.