Since they all have the same name you need to use the elements array to acces them. The elements array holds all of the objects in the form (checkboxes, radiobuttons, text fields, etc).
access it like this:
formName.elements[index];
to see if the item is a checkbox you could
test it this way:
if (formName.elements[index].type == "checkbox")
and then just add a test to see if it selected or not:
if (formName.elements[index].type == "checkbox" && formName.elements[index].checked)
if the above line is true then the element is checkbox that is checked.
Loop through all of the elements in the elements array with:
for (var i=0; i < formName.elements.length; i++)
hope that helps
Dave