To DE test your filter, you simply put a cupful in through the skimmer, and then watch to see if it blows out the returns. If not, check to see if it raised your psi at all. Changing out the sand in a sand filter is very labor-intensive, but not at all hard....but there's no reason to do so unless you have something broken that needs fixing, or have just been through a baq conversion or used some chem that gunked up the sand. Sand never really gets too old to use. In fact, each of the other 3 mods is over 10 years with the sand in their filters, and I would be, too, if I hadn't replaced my filter a couple of years ago....