To verify if any story is true, or spot a false alibi simply present new evidence that is plausible but not true in the form of a question regarding the supposed story. Then simply sit back and wait for the response. Most often, if she verifies your falsity (or worse, changes the subject), she's lying; If she corrects your false evidence with little hesitation, she's telling the truth.
Some people become so good at lying that it becomes second nature to them. Interestingly, we all average over 10 white lies a day. But, when it comes to more serious issues, or you have reason for concern, use this simple trick to verify the truth behind any story. Keep in mind that liars often leave gaps in stories making them abstract. When you want the truth, ask a question that brings up a detail that could be conceivable, but you know isn't true. Lairs will go along with the false evidence, whereas those telling the truth will correct the inaccuracy. Also, look for response cues that could help you determine a truth-teller from a liar. When confronted with a question about a false story, liars will often hesitate and attempt to change the subject matter.