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. .
BIG BAD WOLF: agreed, but in my case, I never challange anyone below me, only equal to me, that does not make me inactive, just in waiting for someone who has earned the privillage of the challange, if someone is above me, then I don't expect them to play me, I have to work myself to their level, ladders in reverse