Example:
telnet 12.34.56.78 22
This will check to see if port 22 is open on IP address 12.34.56.78
An example successful connection is as follows:
telnet 12.34.56.78 22
Trying 12.34.56.78...
Connected to 12.34.56.78.
Escape character is '^]'.
SSH-2.0-OpenSSH_7.4
This shows that you have made a successful connection to the server, if it was to fail or timeout it would look like this and get no further:
telnet 12.34.56.78 22
Trying 12.34.56.78...