How to check if a specific port is open for OUTPUT mode?

I'm playing with iptables on Linux, and I created some rules to allow only port 53 to send packets:

 iptables -P INPUT DROP

 iptables -A OUTPUT -p tcp --dport 53 -j ACCEPT

Now I would like to check if port 53 is actually able to send data to another computer (or maybe I misunderstood the role of a port. Any idea ?

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