How do you know if someone is truly suicidal or if they're just having a passing thought?

You don't always know. If you take some time to talk to that person, sometimes you find out they are lonely and sad and just needed to know someone cared. Or maybe through intervention, they reveal to you why they feel suicidal and by talking to them, you can make a difference that makes them evaluate their lives. However a suicidal person is not thinking rationally or logically, so it does help by realizing that and responding in a calm compassionate way to them. If unsure, help them to find the help they need and alert someone who is familiar with that person and cares to take them seriously. Never dismiss a person who appears depressed or talks about killing themselves. Maybe some people still would be alive if another person had reached out to them in kindness and love. While you may not be able to see all the signs or stop all suicides, there are people who do reach out in some way to show how badly they are hurting.