My friends shows telltale signs of having depression, but I don't want to be presumptuous to assume that she does have the condition. I want to provide her with a safe environment so that she knows that I have her support and she can depend on me. How can I tactfully ask her about this without coming off as 'kaypoh'?
You're a good friend! Sometimes all it takes is just letting her know that you're there for her without really digging into the issues and risking coming off as "kaypoh". For people who are struggling, even answering simple questions like "how are you?" can be quite overwhelming, so it'll be good to just simply let her share when she's ready. A person will only share with someone whom they know is empathetic, non-judgemental and ready to listen, and is not a "fixer".