Check if all checkboxes are selected

Question

I think the easiest way is checking for this condition:

$('.abc:checked').length == $('.abc').length

You could do it every time a new checkbox is checked:

$(".abc").change(function(){
    if ($('.abc:checked').length == $('.abc').length) {
       //do something
    }
});

$('input.abc').not(':checked').length > 0

You can use change()

$("input[type='checkbox'].abc").change(function(){
    var a = $("input[type='checkbox'].abc");
    if(a.length == a.filter(":checked").length){
        alert('all checked');
    }
});

All this will do is verify that the total number of .abc checkboxes matches the total number of .abc:checked.

Code example on jsfiddle.

$('.abc[checked!=true]').length == 0

Part 1 of your question:

var allChecked = true;
$("input.abc").each(function(index, element){
  if(!element.checked){
    allChecked = false;
    return false;
  } 
});

EDIT:

The (above answer) is probably better.

A class independent solution

var checkBox = 'input[type="checkbox"]';
if ($(checkBox+':checked').length == $(checkBox).length) {
   //Do Something
}

The search criteria is one of these:

input[type=checkbox].MyClass:not(:checked)
input[type=checkbox].MyClass:checked

You probably want to connect to the change event.