Assessing Risk To Harm
This is from an APA Continuing Education Tape about risk management and Tarasoff Duty to Protect.

Leon VanDeCreek, Ph.D.--Risk Management

First, he offers that students should keep in mind that Tarasoff is an extreme case, and the majority of these kinds of cases never make it to court. Of those that do go to court, most are ruled in the therapist's favor. When a client acts out under the influence of substance abuse, psychologists have generally been held free of any blame.

Second, determining risk in a case relies on several things:
1
Reasonableness--did the therapist make an effort to assess risk for danger, document this, and review this. You don't have to predict perfectly, but you have to make the effort to assess.
2
Conscientiousness--be careful with clients in crises; return their calls, follow up on missed appointments, don't refer them before they are stable. Don't abandon them.
Do an adequate assessment of risk--inadequate assessment includes
failing to review prior records for evidence of past violence or intended victims

failing to consult with peers or supervisors with more experience

failing to pass along information that would indicate a risk for violence, either to a supervisor or to another clinician
Recognizing risk factors--in assessing risk to become violent, look for
people who have been victimized or jilted

people with a history of impulsive behavior like quitting school or job without forethought

adolescents and young adults

males, as they are 10x more likely than females to be violent

history of substance use

lower IQ

transient mental state like brief psychosis, mania, and recent violent obsessions

unemployment

a specific plan of attack, including a specific individual to harm

possession of a weapon

There is no sure-fired prediction method, but doing this can protect you. Of course, this all must be documented. Things to keep in mind include:
date and time of any interaction with clients (face to face, or by phone), professional decisions you made, and staff you consulted

all record keeping policies should be followed, as failing to do so raises the possibility that you were sloppy about assessing and documenting

make clear distinctions between what a client said, what you suspected, and what you concluded

You are required to make clear the limits to confidentiality.

APA recommended here that you think of these things as having a "duty to protect" which could be met by warning a potential victim, but also by
increasing the frequency of sessions

changing the focus of treatment to such issues as anger management

consultation regarding alteration of medications

including others in the client's treatment

making changes to the client's environment

involuntary hospitalization

contacting police


Of course, good clinical judgment regarding the likely effectiveness of these approaches, and continued monitoring is imperative.

If this makes no difference, consider referral to a more experienced professional. However, never abandon the client.

Bruce Bennett, Ph.D.--Legal Issues
He notes good assessment, education of the public, and some focus on your own safety are good ideas. People can not be divided into "violent" and "non-violent," as many people could, at the right time and place, become violent, and we can't hospitalize everyone who might be violent. We couldn't treat them and can't justify hospitalization based on the possibility they might be violent.