Collapsed collapsible has class ui-collapsible-collapsed
, this class is removed once a collapsible is expanded. Inside a collapsible-set there should be only one expanded collapsible or none.
All you need to filter the expanded one using .not(".ui-collapsible-collapsed")
.
var expanded = $("col-set-id").find(".ui-collapsible").not(".ui-collapsible-collapsed");