How to test for open ports

I'm attempting to setup an FTP server. Along the way I'm finding that many ports which used to be the defaults in years gone by are not working any more, probably because they are being blocked by my (CenturyLink) ISP. So I want to check on which ports would be open for use if setup correctly.

To test for usable ports, I imagine I must do something like this....

Keep the server software running with a designated range of PASV ports, and then input that same range of ports in my router Port Forwarding. Then disable the Windows XP firewall temporarily, and use online port testing sites to check if any of the ports in that range are reported to be open.

Is all of the above correct, or is there a better method?

Most ISPs block common ports (FTP, HTTP, Telnet etc.) in order to prevent you from running your own server. I've had 2 different ISPs in the last decade and learned that although common ports are blocked others are left open. This means you can setup your server on port 8080 or similar. I don't have an FTP server setup right now but I did to facilitate access to some files on a PC I had from anywhere.

I didn't have much success with online port scanners so I just used an FTP client on my phone, forwarded the appropriate ports on my router and tested with my phone. Had it working for years. Recommend you use an encrypted connection FTPS or SFTP since you will be setting up for WAN access. Filezilla server (freeware) supports FTPS and is what I have used.

ANDFTP is a freeware FTP client for Android that I used for years.
Thanks for that confirmation Clippy. I also played with online Port Checkers and never found a port to be open yet per their reporting.

This sounds like a tedious process using a cell phone, and I had already asked about it over at Tom's Hardware. Of course, on that forum everyone will tell you you're nuts for even running XP online, let alone setting up a server.

Do you recall if you had to make any exceptions to the Windows Firewall in the ICMP section?

You need to use your phone because you want to make sure the port forwarding is working. Either that or have a friend test from their home internet if they can connect to your server. It's not as complicated as you think - in ANDFTP you need only specify your IP address, port number, user name and password.

Yes, I did have to permit server in Firewall. I'm not in from of an XP machine so can't specify the settings (Windows 10 firewall is dumbed down).
What is purpose of the FTP server? If its for pernament file sharing and uploading then yeah, find good FTP server for Windows. If its only for ad hoc file sharing (downloading for others) go with HTTP, much easier setup and no problems with FTP data port.