How can I find out what Wifi networks my iPhone knows about?

Where there's a will there's a way ;)

From a stock iPhone this is not possible from the settings. The information that you want is stored in the iPhone's keychain. If you have access to the iPhone SDK you can start messing around if you like, but I will leave it at that since that would be a StackOverflow answer.

Exact location is: /private/var/Keychains/keychain-2.db

I think the only way is to actually be in range of the network that you want to get rid of. You can't actually see what networks you have stored, not that I know of, but if you want to get rid of a network.

Settings > Wifi >

Then select the blue arrow on the right of the network that you are connected to and click "Forget this Network"