In this case, it sounds like the both of you had some mutual comfort in how she dressed, in that both of you trusted both of you to not make a big deal of it. Maybe she thought of it this way and maybe she didn't, but it seemed like she basically trusts you're not too creepy to wear a skirt around without disastrous consequences. She's not going to hold it against you for "taking a peek." But, what will make her panic is if you take it as an invitation (as if she's "asking for it"), or if you publicly shame ("slut shame") her for putting you in such a difficult position.
The way to not be creepy is to not be a threat. Taking a peek has nothing to do with whether you are a threat to her safety. It sounds like the two of you are basically on good terms.