Hacking a wireless access point key

Hi,
I am a beginner in wireless networking. I have an AP(access point router) which is password protected.But I think the key is hacked and someone is misusing the Internet connection.I wish to know the following:

1) Is it possible to know if anyone is hacking into the system?
2)How to get to know the key of another AP router?

Please help me on this regard..

I've actually witnessed someone hack a WEP hotspot (the one in our office, no less). I won't post what he did here, but I'm sure Google will reveal a how-to somewhere. Be aware though, since you're trying to compromise a security measure (i.e., the network's users have an expectation of privacy and integrity), doing so may be a crime where you're based.

fihart is right. You have to find out how to access your router's DHCP table, which is a record of the IP addresses of computers connecting to your network. If you're looking to protect your network, nothing beats changing the password regularly.