Q: How do you know when someone is "mentally ill" and should not have access to firearms?
A: When they have committed a mass shooting.
If the FBI had investigated Nikolas Cruz and taken action to dispossess him, legally, of his AR-15, imagine the outcry from the likes of Rick Scott and Marco Rubio. Imagine the cries of the NRA, of every politician in Florida who has taken money from the NRA, of the President of the US, who was elected with the help of NRA money donated from Russia (you could look it up!). Imagine the cries about jack-booted government thugs rappelling from black helicopter to take away the 2nd Amendment rights of this innocent adult (he was old enough to legally own the gun).
Because until somebody commits an act of mass homicide, how do we know they are "mentally ill" and shouldn't be in possession of a firearm? We cannot jail people for their thoughts or their intentions. I could announce today my intention to commit a criminal act and, if I did, the FBI or some law enforcement might come calling to talk to me, might even open a file on me. But unless I was quite intent on engaging others in my plan (needing supplies, materiel, etc.), law enforcement couldn't infiltrate my plans and arrest me after I had assembled the illegal bomb but before I could plant it; or take me before I left the house that morning with the gun.
And they certainly couldn't begin proceedings to have me declared "mentally ill" (the preferred term of NRA apologists), and even if they did, I could rush to the school and start firing before the first court date.
So what is this crap about "mentally ill" people not getting guns which is meant to protect the 2nd Amendment from being shredded and is meant as the only way we can keep people from firearms in this country? Because it only works in retrospect. Nikolas Cruz was expelled from school, and reports are his mother called the police about him several times before she died. Because he was mentally ill? Or because he was violent? Is violence a mental illness? Is that how we should adjudge who gets a gun and who doesn't? Had the FBI investigated Nikolas Cruz, could they really have done anything? Might they have driven him to act sooner, rather than later?
"Mentally ill" is a dodge, and a cruel one. It indicts people with illnesses that can't be cured by antibiotics, and it presumes people with true mental illnesses are necessarily violent and dangerous. Nikolas Cruz was perfectly capable of planning his violence and carrying it out methodically, right down to his escape. Whatever drove him, he couldn't have done it with easy access to a semi-automatic rifle, and the ammunition to load it. His mental state is relevant to the crime he is charged with; it is not relevant to his ability to commit the crime.
We should not take the question of mental illness and firearm possession seriously. It's not meant to be a serious issue. It's meant to prevent us from doing anything to decrease gun violence in America by decreasing access to guns in America.