Can you view the php files ? They'll usually brag that it's nulled, in the files. Or you can contact phpcow and ask them if the url is in their database ?
search for nulled in the script.. from what I've seen there will be a line like "nulled by somejackass" http://www.google.com/search?hl=en&rlz=1B3GGGL_enUS255&q=nulled+by&btnG=Search
Did a search through all main php files for "nulled" and returned nothing Did a copy for "null" and returned a couple results ie. $cssFile = null; $properties = null; normal stuff I hope?
You can ask PHPCow whether this guy is their affilate or reseller (of course before you should find out the name of that guy). If he is not their partner ( affilate or reseller) then it's not legit.