$('#' + id).is(":checked")

That gets if the checkbox is checked.

For an array of checkboxes with the same name you can get the list of checked ones by:

var $boxes = $('input[name=thename]:checked');

Then to loop through them and see what's checked you can do:

$boxes.each(function(){
    // Do stuff here with this
});

To find how many are checked you can do:

$boxes.length;