You can test whether Deployment Manager is able to reach a target machine (traversing firewall rules, etc.) by remoting to the Deployment Manager server, and pointing your web browser at(or whichever port you chose for machine to listen on).
If you see this error:
System.ServiceModel.Security.MessageSecurityException: An unsecured or incorrectly secured fault was received from the other party. See the inner FaultException for the fault code and detail. ---> System.ServiceModel.FaultException: An error occurred when verifying security for the message.
It can sometimes happen because the clock on one of the two machines is out-of-sync.