If you have to ask, odds are she is.
The best way to tell is to ask her. If she seems angry in a subdued, I-am-trying-to-be-reasonable way, she's telling the truth.
If she gets angry in an outburst of emotion, this is overcompensation for lies, and she is hiding something. Beware as what she is hiding may not be what you think. If you ask her about a lie over the cost of her shoes, and she flips out, she might be lying to you about money, stress, or something even more important.
It is irrelevant that she lied, and more important to stay calm and find out why she lies to you, especially if you would like to prevent if from occurring in the future.