It depends on the type of worms.  Roundworms can often be seen
squiggling around in fresh poo, they look like living spaghetti (and
yes, it's as creepy and disgusting as it sounds!).  Tape worm can
sometimes be seen in the poo and  look more like tiny grains of rice.
More often, you will see the pieces around the rectum.  Hookworm cannot
be seen in the stool as far as I know.  It's always a good idea to take
a stool sample in to the vet if you suspect parasites.  I learned about
retreating the hard way... but it applies mostly to hookworm IIRC.  My
cats are now on interceptor because worms make my Rudy throw up (and
they also freak me out).

I take a stool sample (it should be as "fresh" as possible!) with me for my
cats' routine medical examinations.  Duffy has not had worms since I adopted
him and Holly has not had worms since shortly after her adoption 11 years
ago, but it is still a good idea to have it checked.  Some worms can be
transferred by fleas, and it isn't always easy (or possible) to detect them
visually.

Here are some illustrations of different ways of medicating a cat (and I
would not recommend trying to place a cat "under" you, as Lynne suggested):
http://www.marvistavet.com/html/body_pilling_a_cat.html
http://maxshouse.com/Medicating_Your_Cat.htm

MaryL