The forwarders (client) initiates the connection to deployment-server. This normally uses management-port (default is 8089) of dep-server. Using the same connection, deployment-server will push the apps/packages to the client.
For linux systems,
From forwarders use:
ssh -v -p 8089 <deployment-server>
Also if the connection is established, you can see the connection info in
index=_internal sourcetype=splunkd source="/opt/splunk/var/log/splunk/splunkd.log" ip=<ip_of_forwarder>