IF it's eval edition, you can get info here: http://mangalpardeshi.blogspot.com/2009/05/expiration-date-of-sql-server.html

If you want to query each server, I'd think that eval edition had a note in the @@version that it was the version installed. It's also probably in the registry somewhere.

Serverproperty is supposed to return this: http://msdn.microsoft.com/en-us/library/ms174396%28SQL.90%29.aspx%5B/url%5D