there are several way :
var isChecked = $("input:checked").length > 0
var isChecked = $("input").is(':checked')
var isChecked = $("input").prop('checked')
var isChecked = $("input").attr("checked") === "checked" // WRONG (dows not change if you check/unckeck it)
and a lot more.
i prefer the second way, it is a lot easier to read
read more about them here
http://api.jquery.com/prop/[
^]
NOTE : your .prop("checked",true) is a SETTER, you are checking the checkbox, the GETTER has no value (omit the true)