As a person who has been suicidal for years(currently diagnosed with severe depression), I feel like lots of people know that something is wrong and they just choose not to do anything about it because they don't want to overstep their boundaries. For instance, my mom asked me about some razorblades in my room that she had found, I said it was nothing and she left it alone. I always wished that someone would pressure me about it rather than shying off when I pushed them away.
Lately I feel like I've undersold my depression. I never wanted to burden anyone and everyone just abandoned me because of it. Maybe if they thought I wasn't just feeling sorry for myself they'd have done something about it.
Long story short, don't give a fuck about what they say. If you feel something is wrong with someone, ask them about it. Really, FUCKING ask them about it. Don't take no for an answer.