It may happen that you're working on a server with a user that has very limited capabilities. Sometimes you have no access to
telnet:
telnet host port
nmap:
nmap -A host -p port
curl - it will complain that it did not receive data, meaning that it connected successfully:
curl http://host:port
netcat:
nc host port
or whatever other tool you like to use. So what to do? Well maybe, you have access to bash:
cat < /dev/tcp/host/port
but I find this method not very reliable. Instead you might have access to Python as well:
python
import socket
test_conn=socket.create_connection(('host',port))