In addition to janitors & wait staff: you can chat with the homeless people, too. You don't have to ask them about "their situation" or offer to help them. Weather, sports, the traffic... just treat them like a person who's not invisible.
Before anyone jumps up & objects: yes, there are a lot of homeless who don't want to talk to you, or who are downright dangerous. You can avoid those.
How do you pick out someone as being homeless? I'm in the UK and (accepting in the US it may be different?) there is a common misconception that people who sit in shop doorways begging for money are 'homeless' people when, in fact, they are usually just people who have a drug habit they need to continually finance.
The real homeless people are doing their best to blend into the background and not to look homeless.
Before anyone jumps up & objects: yes, there are a lot of homeless who don't want to talk to you, or who are downright dangerous. You can avoid those.