IMPORTANT: if you or someone you know are having suicidal thoughts or feelings, immediately contact your local emergency services!
A suicidal person may or may not self-harm. It is a very serious warning sign that might need help. The body tends to slow down, possibly to keep oneself from harming themselves.
In assessing suicidality, a professional would normally determine if there is a plan, it's lethality, means, and the seriousness of the threat.
Canada, United Kingdom, and United States: http://www.suicidepreventionlifeline.org/
Cayman Islands: http://www.gov.ky/portal/page?_pageid=1142,1593314&_dad=portal
New Zealand: http://www.lifeline.org.nz/corp_Home_378_2001.aspx