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. .
srnity: No..you dont need to "ask", but if you challenge an opponent who has "no autopass" in settings than you cant use it either. So in fact its your opponent who decides if your settings are in use or not....fair?...ask Fencer :)