Traditionally, you would show an animated gif when doing ajax call to let user know that they have to wait to see some results. We do this by showing and hidding the animated gif in a div for example.
Actually if you are rushing for time, a quicker and simpler solution is to change the mouse cursor. Not that fancy but it works. Users should be familiar with different types of mouse pointer. In the jQuery.ajax function for example, we can do something like so:
and after the ajax call