So how do goppers propose to evaluate the mental health of gun owners/buyers?

by doing nothing

Gun Buyer) I would like to buy an AR-15.

Gun Seller) Sorry you failed the mental health test because if you want to buy, own, or shoot
an assault rifle then you do not have the mental soundness to own said weapon.

Good plan. Kurt Vonnegut also advocated its use for holding public office.

Congratulations, Citizen! You are not only stable, you may even be a genius!

Any test they proposed will be designed to keep guns out of the hands of people they don't like

Many will contend, that because of 2A, that no one should ever be denied the right to purchase, carry or own a firearm, and that any attempt to regulate firearms is illegal and that all current regulations are in violation of the Constitution. When pressed hard, they will also claim that if the government has nuclear weapons then nothing should prevent private citizens from the same. This is not a theory, I have had the actual conversations!

You can easily google how to pass a psych eval or suicide assessment. No doubt we need better mental health care including better wages for those who do this work in the community but to expect this to be the only answer is just idiotic.

They must be declared "legally sane"

In other words, it's BS It will never, ever happen.

There are already laws in place for those who have a "legal history" of mental health issues. And by legal it means: "Mentally incompetent", not someone who has mental health issues.

Just more stall tactics to run the clock out.

including the Constitution?

No?

Then I'm afraid I can't sell you a gun."

some kind of vaginal/uterine examination....