Verbally and non-verbally indicate you genuine concern. Involve the person in a suicide contract. It sounds strange, but it works. Refer the person to an appropriate agency for assistance. And be willing to accompany the person to the initial contact session.
Intervention is one aspect.
You may come upon someone who has attempted or completed suicide. If appropriate, render assistance by aiding in stopping bleeding, untying a rope, etc. In all instances, contact authorities for assistance. Off campus - 911 for police emergency. Be certain to give the dispatcher complete information as to the location of the incident, your perceptions of what has happened and await their arrival.