Ask first. If she's cool with it she'll let you know. If she is not then you have avoided an unnecessarily akward situation. Either way, you will make her day by showing her respect and concern by asking first. The whole situation will be much more positive and she may be excited by the chance to see you en femme.
I would take the ask as a huge compliment if I were in her shoes, and she will recognize how much it means to you too.