Is a friendly app that is on almost all Linux machines and many windows machines that you can use for some basic connectivity testing.

telnet ip port
Examples - suppose is the server

Test for RDP enabled

telnet 3389
If telnet is able to establish any connection then this machine is most likely a windows machine and has RDP enabled

Test for SSH

telnet 22

Test for SMB (Simple networking under windows)

telnet 445
telnet 139

Type quit to exit, but if you get stuck in a terminal transmission session then you might have to end the command with ctrl+]