Try to ask everyone involved in the clan to log into Elite and see if the option, 'manage clan' appears. If one person can see this option, they are Leader. However I'm not sure if a leader can leave without appointing a new one first, so if nobody can see the manage clan option, it may be neccessary to leave and find or make a new clan. Leaders shouldn't just abondone a clan as it clogs up the system with so many inactive clans that have been left, Im pretty sure there's always an option to delete the clan or pass it onto somebody else.
go on to mw3 load up MP then press Y/Triangle to view friends scroll over to the left to elite clan who ever has the full Clan tag next to there name with out hovering over it will be your leader
Like so. im x iAiDEN x the leader of uGoTDominated.