Hi
Do you mean a port opened that allows traffic through the router ?
If so then unless you have an access-list applied to an interface that denies certain ports by default a router will allow all traffic through so the easiest way is to do a "sh run" and under the interface statements check if there is an
"ip access-group "no/name" in or out
If there is then you need to look at the corresponding access-list so for example
interface fa0/0
ip address 192.168.5.1 255.255.255.0
ip access-group 101 in
If you found this you would then do
"sh ip access-list 101" to view the access-lists.
HTH
Jon