ping does not use ports, that's UDP and TCP within the TCP/IP model. you can't port forward somethign that doesn't know about ports. what i'd probably suggest is not using pings at all, if you do a port forward on your external device to a known service on each internal box, then use a tool like netcat or nmap to try to open that port. if, for example, you have an ssh server on each host, port forward each from say, port 2201 2202 and 2203, then run nmap against those port numbers "nmap host.com -p2201,2202,2203" that will show if each port is successfully port forwarding and therefore the internal box (AND the service) is running. alternatively, try netcat. run "nc host.com 2201 -w1" this will actaully connect to that port and show you the id string it recieves, so sticking with the ssh example, that would return something like "SSH-1.99-OpenSSH_3.9p1" showing that the remote service is OpenSSH etc... obviously exposing SSH to the internet is a generally dumb idea, but that's just an example.
also you could just run a web server on each box, and pull down a basic index.html page via curl...