I've always relied upon SELECT @@VERSION, because the version to which a client can connect is more important to me. My second choice would be to inspect the errorlog, after SQL Server has been bounced, because that confirms I restarted the correct instance, with that errorlog reporting both the instance name and the build number. As a sanity check, you might want to try one or the other of those. Last I checked, SSMS reports the RTM (original) version, not the service pack build number.