Thing is, if you don't let her know and she does like you, you might miss a chance to become more involved. I know I've banged my head against the wall on several occassions when I ran into girls I knew from high school or college who said they had wished I'd asked them out. These were girls I'd have loved to dated back then, but had no idea they wanted to date me as well.
I'm fairly certain she's interested in me, which is a rare thing for me since I'm so bad at reading a person's intent. For instance, she's not a very touchy feely person, and I'm next to certain she knows that I'm not the type of person that is either. Let me put it this way- I grew up in a family with no higs or kisses, save from my grandma, no warm affection, and my time in the army nearly completely solidified that mentality. I'm simply not comfortable with people touching me. She has lately made it a point to try to break through that- grabbing my hand or arm playfully, or hugging me at random times, which is somewhat out of character for her as far as I can tell.
Yeah, I'm fairly certain she's interested, even if it isn't on a conscious level, but I made this decision just the same. The biggest problem is that I'm also fairly certain that there are certain things she wants in this life- marriage, kids, little suburban house, you know, the works, and these are things I'm not offering. I've also noticed her marked silence whenever I drop a hint like "and people wonder why I don't want to have kids." I can't just be using the poor girl when I know full well I'm not going to giver her what she wants.