Instructions
Turn off the Xbox 360. If it's not already connected, connect the Ethernet cable into the "Ethernet" port on the back of the Xbox and connect the other end into the "Ethernet" port on your broadband (cable or DSL) modem or router.
Turn on the Xbox. Select the "My Xbox" tab. Select "System Settings."
Select "Network Settings." Select "Test Xbox Live Connection." If your Xbox is up to date, the test will begin, and you will know whether or not the Ethernet adapter works.