You can do a simple curl request to the FQDN/IP of the ESXi host on port 902. If the port is open, you should see something like
curl esx5.domain.com:902
220 VMware Authentication Daemon Version 1.10: SSL Required, ServerDaemonProtocol:SOAP, MKSDisplayProtocol:VNC , VMXARGS supported, NFCSSL supported/t