I've only got a couple from experience...
1) if she's constantly making excuses to not see you
2) if she "admits" to cheating on you, and then later admits to lying about cheating to see your reaction. (yes this actually happened to me, she was trying to get me to break up with her, and when that didn't work, she cut the crap and just broke up with me directly)
3) If she doesn't want to touch you. I'm not specifically referring to sex, rather she doesn't want you being affectionate with her at all. However, this doesn't necessarily constitute a red flag by itself; many of the people here, both men and women., are touch-aversive in general. However, if she hugs her friends all the time but still doesn't want you near her, then there's a problem...