When the mouse hovers over each column I'd like the tooltip to indicate whether that column is sortable.
I am able to change the title attribute with something like this:

$("#List .ui-th-column").each(function(i) { 
    var isSortable = i % 2;
    $(this).attr('title', isSortable ? "Not Sortable" : "Click header to sort."); 
});

I'd like to replace the demo expression 'i % 2' with a check of the colMode's sortable property, but I can't figure out how to get the value of the colModel's sortable property.

colModel: [ { name: 'Name', index: 'Name', width: 100, sortable: true  },
            { name: 'Note', index: 'Note', width: 200, sortable: false } ]

I've tried .getGridParam and .getColProp but I don't think the syntax I'm using is correct.