When they go out of their way to engage you it is an IOI.
I run a state park regularly and I always say hello when I pass runners going the opposite way. When I get down to the more travelled sections the people are too numerous. So I have a pretty good baseline of gauging interest.
After a run a girl made eye contact, and we said "hi" She followed with "You're cute..." Yeah it was on...
If I'm standing still and they stop, that's interest in an introduction...
If they return the greeting but keep moving it's just being polite.
I've had a mountain biker stop and chat me up along the trail.
If they follow up the greeting with a comment there is at least some platonic interest.
If they look back after passing, there is some curiosity at least. You can engage with "You look familiar, have we met?"
Now it is going to be different on a NYC street, than in my little community or on a park trail. You have to calibrate it by pausing, and if they pause deliberately it's a pretty good indicator of interest.