Could it be that someone is depressed but she is hiding it? how to know if someone is under depression?

Not always possible: I'm glad that you care about those around you. It's not always possible to tell, and in fact, if a person decides to embark on a fatal course, the mood may even seen to lighten. Perhaps the best is to say, "you look sad." or, "you seem sad, is there anything we can do?" any talk of self-harm, consider yourself phoning a suicide help line for guidance.