I don't know if you are joking or seriously asking, either way...
I have noticed that a chicks asshole has a look about it when she either does a little anal play(finger/dildo) or straight takes the dick, kind of like an indicator. Either her asshole as a gap(little or big) you can see or if not the gap they have what I call an indention, sometimes both. Indention is best seen when they are in the backhot position, you can see the gap for the most part when her asshole is in view.
I could be wrong but using the pics in this post I think all these checks are down with some anal play.
Indention. The last two have a little gap also.
Indention with gap.
Gap, more visual cause her ass is being spread.
Sometimes her asshole just looks roughed up like she has never used any kind of lube.