I think this is well worth a read. [I am an Admin at this site, but I think this information is invaluable]

Please remember, prior to every match to check what the medical presence is. And also remember that as the referee, your judgment on whether a player is fit to continue is the one that matters. Nothing trumps player safety.

http://www.rugbyrefs.com/content.php?333-The-importance-of-recognising-suspected-concussion