how to detect spoofing hosts

Hello! Let´s say i have a big LAN, with a few gateways and a firewall. As bigger your networks get´s the more you will be conflicted with pepole who want to "explore" your LAN. Thats why i was wondering how you can detect spoofing hosts. And why can spoofing be so dangerous, i couldn´t think of an good example. Thank you! Spiekey