1. She Can't Answer Questions -- Often, people don't think through lies before they tell them. For example, if your friend is trying to tell you that she couldn't make lunch because she was having lunch with someone else, you can easily out their lie by asking for details. Where did they have lunch? What did they eat? How long were they there? Many times, people get nervous when asked questions, and if you ask lots of questions, they think you know that they are lying.
2. She Gives Too Many Details -- People have started to figure out that telling a poorly-thought-out lie gets them caught, so they begin to make up wild tales when they are lying. When most people are telling the truth, they don't feel it necessary to expound upon every detail of their day, but they also don't get nervous when they are asked questions. It's a fine line.
3. There Is Inconsistency -- One way that you can improve you ability to tell if someone is lying is to listen closely. Most people only listen with half an ear as they wait for their turn to talk, which doesn't help anyone. Instead, listen to everything someone says, and try to pinpoint inconsistencies. For example if your friend insists that she was visiting her relative at the hospital during your party (she'd promised she'd be there), but your party didn't start until nine o'clock and you know that visiting hours end at eight then you would notice that inconsistency.
4. They Get Irritated -- When people feel as though they are being backed into a corner, they get irritated rather quickly. A casual conversation with your boyfriend or girlfriend shouldn't result in snide remarks and irritation; that's a sure sign that someone is lying. The same goes for someone who cuts off a conversation on the phone or who changes the subject quickly. Most people don't get irritated by questions if they're telling the truth.
5. She Turns It On You -- Most people who tell the truth aren't rattled when someone accuses them of lying. They know that they're telling the truth, and they assume that the facts will speak for themselves. For example, if you continue to question your girlfriend or if you let her know that her story doesn't make sense, she gets immediately angry and says something along the lines of, "You don't trust me!" or "How could you accuse me of that?"