Ok, if you set up the ip address of the middle machine correctly, you can use the PING command to see if the middle is alive in your LAN. If you ping the ip address of the middle machine and it replies you with the following information, it means the computer and the middle machine connect successfully.
And when you input the ip address of the middle machine in a browser, if it can shows up information likes following screen, it means the testers connect with middle machine correctly. From the following screenshot, we can tell there are 2 testers connected with this middle machine. And you can see other more information of the middle machine.