When my depression was at its peak and people would politely ask this question I would always plaster on a grin and answer, "I'm fine."
The other popular answer was "Just tired."
When I really felt like this:
Now, I don't word vomit my problems all over unsuspecting friends. But I don't smile and say "I'm fine" when I'm really not.
After my surgery a friend checked on me to ask how I was doing. My response was something like, "Sore, tired, bored." She told me it was refreshing to see me honestly say how I felt. I didn't see a reason to lie. I was bored as hell staying in bed. I was tired because healing is exhausting. I was sore because I had surgery. Why bundle all that up in "fine" when I wasn't?
If I'm having a bad day I'll give it a word like "overwhelmed" or "stressed." This is good for ME. Instead of pushing my feelings down and ignoring them, I give them names.
There are times when a friend will hug me and ask how I'm doing and I'll respond with "Okay" because I'm not horrible but not GOOD either.
And if I don't want to talk about it?
I don't hide that either.