A bunch of tiny little worms, pin-shaped, and 8 - 13 mm long. But often this isn't the case and the perianal area appears normal or a little irritated from all the itching. In this case consider confirming the diagnosis with the "scotch tape" test.
- press scotch tape against the perianal area
- put tape on slide and (have lab) examine it under a microscope for characteristic bean-shaped eggs (image below)
- repeat testing may be necessary to increase sensitivity; a single specimen detects approximately 50% of cases and three swabs approximately 90%
PS. Sorry for the large indecent picture above. Hopefully you were not reading and eating.
Image, pin worms: http://abyssaldepths.files.wordpress.com/2008/04/enterobius-vermicularis-pinworm.jpg
Image, eggs: http://www.legacyhealth.org/images/Housecalls/pinworms.jpg
Leder, K. Weller, P. "Enterobiasis and trichuriasis" Up to Date. 13 June 2008.