How do I know if I've been bitten by a black widow spider?

Well, the initial bite is painless, but you'll develop a mark like this pretty soon:

This is very painful in the short term, and followed up by these symptoms:

swelling, reddening, nausea, stomachache, muscular spasm, thorax cramps, abdomen and leg cramps, as well as contractions and deep general pain.

Check out this site for more. The only treatment is a fast trip to the Emergency Room.