How to determine wear on a stylus?

Over the years I've accumulated several cartridges and would like to determine which have styluses that are still in good shape. I suppose a microscope would be the best way to examine them; any recommendations as to type and power? Or any other methods of determining wear on a stylus?
ahem, ah when it sounds dull???
RadioShack has a small pocket scope that works well. It is lighted and you can actually see the stylus (if you can FIND it at 100x!!!)
I search for the cantilever and then move up slowly.
Thanks, Elizabeth. That sounds like it's worth a try.
". . . when it sounds dull??" My concern there is that by that time it may have begun to damage the records.
A microscope w/ 200X magnification. Some good audio stores had them prior to the CD onslaught. One would think w/ the mega buck T/Ts, their dealers would invest in one. After several stylus replacements or retippings, it would pay for itself.n
The stores that had a prominently displayed microsope did not necessarily know what to look for. In this second coming of vinyl I would argue that the only safe way is to keep track of the hours played and change it when a reasonable number is reached. I fondly remember carts with a user replaceable stylus where you could actually do something about a worn stylus short of forking over as much money as you paid for the cart for a retip.