You've only got one of two options here if she's asking you about her apparel. Either she feels damn sexy in that shirt, those shorts or whatever else it is she's wearing, and she wants you to confirm how good she feels. Or she's not sure if what she is wearing is worth the discomfort of wearing it.
Either way, you're answer should be, "Meow!" (and then make a genuine compliment)
Absolutely, whatever you do, DO NOT say, "You look good in anything babe," or any other cliche's of that nature. You'd be better to offer her a shirt you would like her to wear, then to use a tired cliche.