How do you know if someone has depression?

[c=#BF0000]I know this person and they've been acting a bit different than they usually do and I've been noticing that they wear a bandage around their ankle and won't let anyone see under it saying "it's just some really bad scratches from the cat." I've gotten really suspicious of how they've been acting, can someone please explain how I can tell if she's depressed?[/c]