How can I know which ports are open in remote servers?

Question

Specifically I would like to know the FTP or Telnet or SSH ports that are open on the servers.

Answer 1

use a port scanner
ftp is port 21 and ssh is 22 - defaults
if you know the ports you want, u can just try telnetting to them too

Answer 2

FTP is usually port 21.
Telnet is usually port 23,
And SSH is usually port 22.

For the rest, you could use a port scanner to find if the port's open.