That's my point. How can I tell the difference?

One of my ideas was legitimately to just hold a giant sign that just says "hi"

I had this idea a few months ago, but never wanted to waste a whole Sunday on it. Maybe I should.

To me that seems like not saying hi, rather a device to shift the risk of engaging off of you. Don't be scared. Just say a few banal words to people you don't know every day and gauge their reaction, start a conversation if you like and they seem to be up for it.