It's not difficult at all!
Prior to passage of the Community Mental Health Act of 1963, it was routine. It must have been easy and done routinely because mass school shootings only began in 1966, with Charles Whitman at the University of Texas. Before that, almost everyone who was demonstrably bat**** crazy was committed to a psychiatric institution.
Normalization of abnormal behavior began in 1963, continued to the American Psychiatric Association voting that homosexuality was not a mental illness in 1974, and continued to 2018 when someone who has the police called to his home 39 times, posts pictures of tortured animals on the internet and has the FBI called on him twice for threatening a school shooting doesn't raise any eyebrows.
It's all "defining deviancy down." Everyone is a unique and precious little snowflake today.
Well I think you are saying that so called “ mental illness” is ambiguous and undefined.
It could be that after a jury trial, this guy should not buy a gun but that will not be just deemed by so called experts. That is a slippery slope beyond comprehension. I assure you that “deemed mental illness” in unconstitutional as hell and would end up putting Christians, “global warming” deniers and conservatives on the “list” . The constitution stops that cold. One great thing to compare with is legal guardianship for mentally incapacitated . In order to deny that person’s rights there is a full blown court trial with Doctors testifying etc. Notice and hearing due process. The ward in that guardianship proceeding is not only represented by separate counsel, but that counsel MUST resist that petition for incapacity vigorously. We are talking about constitutional due process when we attempt to deny ones rights and the media and lynch mob mentality today is in gross denial of that reality.