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. .
WizardII: Aye. Instead of grabbing the legs of those above and trying to clamber over them it's more a question of seeing who's below you and stomping on their hands as they try to reach upwards (or booting your peers off the platform). ;-)