Looks like you must enter in
game keys, so for every football league (plus or free) you can do:
CODE
http://fantasysports.yahooapis.com/fantasy/v2/users;use_login=1/games;game_keys=57,49,79,101,124,153,175,199,222,242,58,62,78,102,125,154,176,20
0,223/leagues
Or a shorter version, it seems passing in the name instead (e.g., 'nfl') will get the current league info. So:
CODE
http://fantasysports.yahooapis.com/fantasy/v2/users;use_login=1/games;game_keys=nfl/leagues
Maybe one day I'll look for a "Find all leagues this user is in, of all types" but maybe later as it's trivial to store/know all available yahoo sports game keys.