To determine whether the lobby that launched your application supports standard lobby messages, it is necessary to send the lobby an initial DPLMSG_GETPROPERTY request and see whether it responds or not. The property that should be requested is DPLPROPERTY_MessagesSupported.
If the lobby responds with TRUE, then you can assume that it will respond to all further messages. If it responds with FALSE or doesn't respond at all within a specified timeout, then you can assume that messages are not supported.
If the application supports spectator players, then the application should also request DPLPROPERTY_PlayerGuid before creating players to determine if they should be created as spectators or not.