You never know how another person is feeling.
Yeah, they're doing good, and you? -- don't always believe that. Every one of us always has more going on mentally than just "good," most of which you'll never know about no matter how much you talk.
Aside from digging deeper and maybe getting to what it actually is, what you can do it offer and kind word. Something positive to another person that may be the lifeline they need to stay in good spirits. Or and idea that reminds someone of a reason they have to be happy.
You never know what one kind word can do for another person.