I know because the girl tells me. Also, I knew before she told me (my current gf). Also, I said: "Listen woman, I like you like that and that's just the way it is." And then she told me what she had been feeling.
I don't think guys really send out signals so open for interpretation like females. I don't know Kev's full situation because I haven't read the whole topic yet, but girls love a guy in the friend zone. And they love to keep them there.
You asked, "how do guys know?" SIMPLE: The girl shows genuine interest in him. She'll ask a question deeper than "Why you so stupid?"
She can be shy/flirty/etc. but I don't wanna write too much because I'm a busy guy who has class and work to go today and I don't want nobody getting the wrong idea thinking that I ain't busy (a,b,c,d,e,F, U!).
God Blessa!