how would I know if I needed a timing belt on an 02 chev cav?

If it's never been replaced, now

But wait, according to http://www.survivalistboards.com/showthread.php?t=222705 and other places I looked it has a timing chain

http://www.gm.ca/media/owners/manuals/2002_Chevrolet_Cavalier_Manual_en_CA.pdf /has a maintenance schedule, Chapter 7, and no mention of a belt other than serpentine

FordNut is right. No need to worry about it any time soon. Especially since you only have 100,000 km on it. That's only like 60,000 miles. I wouldn't worry about it until its got at least 100- 120,000 miles or 180- 200,000 km on it.

The 2002 Cavalier has a timing chain, so no service schedule. You only change anything, such as a tensioner and chain when you hear noise.