Here’s an extension attribute script to check for the Microsoft Silverlight version on Mac’s via an Extension Attribute script in Casper, copy and paste and away you go:

if [ -e /Library/Internet\ Plug-Ins/Silverlight.plugin ]; then
    result="$(defaults read /Library/Internet\ Plug-Ins/Silverlight.plugin/Contents/Info SilverlightVersion)"
    echo "<result>$result</result>"
    echo "<result>Not Installed</result>"

Thanks go to Ben Toms aka @MacMuleBlog for this Extension Attribute