Thought I'd shared one the secrets of passing in guy-dom: "The Nod" -- the simple up/down head nod that men do to each other to acknowledge each others' presence without speaking.
The subject came up on another forum and someone had an eloquent description of it.
Kind of hard how to explain how to use it properly, the best I can suggest is just doing some field observation of the natives.The nod can mean, everything's cool, hello, goodbye, this is my woman, this is a girl I am using for sex, I can kick your a$$, you can kick my a$$, and a lot of situation specific things, but mostly it is just an acknowledgement. I am not sure why, but men must acknowledge each other if they have eye contact. It may be a territorial thing.