Go to https://www.yougetsignal.com/
Click "Port Forwarding Tester"

Yougetsignal.com will automatically populate the public IP address.
Enter the "Port Number" you wish to scan.
Click "Check"
Note: You may also manually enter the "Remote Address" to run a scan on a remote network.

If the port is open, you will receive a messaged saying the port is open, as well as which IP address.

If the port is closed, you will receive a message saying the port is closed, as well as which IP address.