You know you're a faggot when...

you use Arch Linux
you have a repo at Github where you share your `dotfiles'
you use Github
you have friends at Github
you comment on Hacker News
you use Reddit
you post screenshots of your Desktop at /g/ (dwm, terminus font, no antialiasing, a couple of semi-transparent terminal emulators with mpd, weechat and the Arch logo in ascii)
you believe that HN is not only mind-opening but will eventually make you a richer person
your editor is vim, sublime text or textmate
you used vim, sublime text or textmate to edit edit .Xresources and spent hours creating a `color scheme' with faggy pastel colors
you forked the solarized color scheme at github
your shell is zsh
your shell is zsh and you use oh-my-zsh
you love the Ruby programming language
you love the Ruby programming language community
you love node.js
you should kill yourself

That depends what you mean by faggot.  If you mean in the most literal sense where faggot is a derogatory term for a male homosexual, then you "know" when you realize you are sexually attracted to men. 

If you mean it as a catchall term for a male person that you, or a society finds unpleasant. Then that of course depends on your and/or the societies preferences.

Also OP and others, may I remind you that it's 2013 and you shouldn't use gay slurs for a number of reasons.   Imagine the hardship that gay people go through in life for a second next time you feel like using gay slurs.  Maybe if you had some experience being looked down on, excluded and ashamed of something you have no control over,  you'd be less cavalier about using language that propagates the cultural sentiment of treating gays like the lesser.