To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in. .
Andersp: Yeah, which makes autopass a pretty useless feature if you ask me. I've set my setting to autopass as soon as it was possible, but so far, I've yet to encounter an opponent that lets me play with it.
Perhaps I should change strategy. Whenever I have to use pass manually I wait till just before my time runs out. ;-)