So I thought I'd add my two cents to this thread, because I find it interesting. Not sure what I have to say is worth that much though.
I'm completely oblivious. To everyone. It's not a matter of not knowing if a lady is femme or not. It's me being so lost in my own head and probably thinking about my own feelings of invisibility that I don't notice anyone around me in return.
And when it comes to a femme being interested in me or flirting with me? I'm definitely the butch you'll need a 2x4 with. Unless you make it 100% clear and obvious, I will not pick up on the subtly.