Iptables rule to allow only one port and block others

We can make INPUT policy drop to block everything and allow specific ports only

# allow established sessions to receive traffic
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# allow your application port
iptables -I INPUT -p tcp --dport 42605 -j ACCEPT
# allow SSH 
iptables -I INPUT -p tcp --dport 22 -j ACCEPT
# Allow Ping
iptables -A INPUT -p icmp --icmp-type 0 -m state --state ESTABLISHED,RELATED -j ACCEPT
# allow localhost 
iptables -A INPUT -i lo -j ACCEPT
# block everything else 
iptables -A INPUT -j DROP

Another question, would this be the right way to test, or maybe I should use "netstat" command to see which port has connection established with the other ip?

Yes, you can check netstat -antop | grep app_port and you can also use strace :

strace -f -e trace=network -s 10000 PROCESS ARGUMENTS

To monitor an existing process with a known pid:

strace -p $( pgrep application_name) -f -e trace=network -s 10000