How do you know if a person is a sub ?

[Sub William]

Well I usually drop subtle hints, and some women pick up on them. Most of the time the hints are completely missed!!

[Kaori-san]

When I was a high school in grade -thinks- 9 my best friend.. and still my best friend was definatly a dom.. and I'd know I was a sub ages before then.
Most people in my year had no idea what bdsm was, but us, we weren't the most innocent girl ever we often had fun emailing pictures of naked men lolol (mainly hentai hehe)
We are both definatly straight as she had a boyfriend, as did I and you only had to show one of us a picture of a hot guy and well.. we were off pointing out what exactly made him hot
But I could always tell she was a dom, and shes always know I was a sub, I think its just because you can tell if you become friend with someone, we never said anything about it much, we just knew. Trust me, if you get to know someone.. you can just tell