Experts have come up with an acronym for helping those who you may feel are suicidal. It's called P.L.A.I.D P.A.L.S and means:
Plan: Do they have one?
Lethality: Is it lethal?
Availability: Do they have the means to carry it out?
Illness: Do they have a mental or physical illness?
Depression: Chronic or specific incident(s)?
Previous attempts: How many? How recent?
Alone: Are they alone? Do they have a support system?
Loss: Have they suffered a loss? Death, job, relationship, self esteem?
Substance Abuse (or use): Drugs, alcohol, medicine? Current, chronic?
Although someone does not need to fulfill all these criteria to be suicidal, it can be useful if you should find yourself in a situation where you are unsure of a friend's mental state.