Ubuntu :: Run Script Only When On Wired Network

I have a script which runs periodically and copies some very large files from the server to my laptop. This works fine, except sometimes my laptop is not connected to the wired gigabit network. I want to prevent the script from running while on the much slower wireless network. Is there a conditional I can use in the bash script to test for wired connectivty? Something like if [ "$eth0" = "on" ];