I'm going to tell a girl that she's not really suicidal.
Did she say that she was feeling suicidal?
Yes, I think she's just trying to get attention.
It's not your place to tell her how she really feels. If she said that she is having suicidal thoughts you should be supporting her, not doubting her.
It's so annoying that people fake being suicidal.
It is wrong to assume someone is faking being suicidal.  She needs to get the help she deserves, and you are not helping.