What one finds frequently with girls who specify anal at discretion, is that the rest of their likes list is completely vanilla and restrictive.
Because the word anal starts with an 'A' it's at the top of the list, and immediately gives off a signal that the girl is hot.
When you actually get there she can simply say she's not going to do it, and then your thrillingly naughty encounter turns into an hour of boredom in the space of a second.
Always look for reports of anal in their reviews, to see whether they actually have any intention of doing it.
Equally though, especially for the uninitiated, if you're looking for anal penetration with a girl who genuinely might, spend some time gently teasing and probing her arsehole with your finger, and ideally your tongue, to get her in the mood.