If there is no selection, you can use the properties .selectionStart
or .selectionEnd
(with no selection they're equal).
var cursorPosition = $('#myTextarea').prop("selectionStart");
Note that this is not supported in older browsers, most notably IE8-. There you'll have to work with text ranges, but it's a complete frustration.
I believe there is a library somewhere which is dedicated to getting and setting selections/cursor positions in input elements, though. I can't recall its name, but there seem to be dozens on articles about this subject.