In most cases, I have to be physically attracted to my victims. If she's wearing a dress or skirt, and especially pantyhose, then she can quickly move up the attraction scale. Dark hair, nice make up, high heels, and a nice laugh will also move her up. If she looks at me and smile - she's on my list.
Going by that "criteria" I'd say any woman I'm not attracted to is safe. That said, I should preface this by saying that I will often see a woman and not be attracted to her, but seeing her again may change that. I guess it might mainly be the way she's dressed.