You can never know for sure. You either trust or you don't And that goes for men and women. Judge by their overall character, do they lie to others frequently, then they probably lie to you also. Then decide if what they are lying about is truly a big deal. Small, inconsequential lies are common, but it depends what your willing to put up with.
You have to judge her actions and facial expression while she speaks. You will know yourself by her actions and the way of talking. If she suddenly turns her head away from you after she is done with speaking, she probably is lying. And when they lie they don't seem to appear confident and tries to change the topic .
It depends on how good of a liar someone is. Different people also might have different habits on what they do when they lie like looking down at the ground instead of looking you in the eye, or their face turning red. If she answers a question with a clear answer that she seems sure of in the way she says it, she is usually telling the truth. If she ever hesitates, she may be lying. If it's over text, take into consideration the time it took her to answer you. If it took a ridiculously long time to reply back to you, she could either be in the middle of doing something or thinking of something to cover up a lie. You can never jump to conclusions though. Always make sure to be honest with her, because over time, she will learn that if you trust her enough to always be truthful, she will be honest with you.
Sometimes you can't tell if a Female is lying because we do it so good lol jk but it is harder to tell if a female is lying then a guy because the guy will act totally different then normal most girls can act normal like nothing happened