Forget how you can tell - here's how you can help

Less important than knowing whether she is or is not clinically depressed is what you are going to do about it. Whether she is suffering from depression or just normal teenage angst, odds are she won't be talking about it with her parents.  If there is an adult in her life whom she trusts (e.g. a teacher, older relative, etc.), you may want to bring that person in to connect with her. If not, you could gently offer her "someone to talk to" and help her find an appropriate counselor (in person or online).

That said, when it comes to depression, big changes are the key to look for.  If she drops all her old friends, plummets academically, changes her activities - that could be a sign that something's up. Again, there may be little you can do about besides providing a supportive environment - which may mean NOT asking her what's going on every day - and gently helping her connect with a professional who can help.