How to find nether portals in MC?

I have a role playing server in Minecraft where nether travel is banned because of a virus. In my server i allow rebels to go to the nether but if they are caught they will be punished. What i have been wondering is "Is there a way to find all the nether portals on a server

Guest

There's no way I can think of to detect nether portals other than using a command block to "testfor" the nether portal block (the purple swirly things) but that would be cheating… So the only other way would be to listen for the portal sounds.

Harry

Nether portals are generetad in the "normal" minecraft world when you enter one in the nether world, if you enter a portal that you came thru then it will not work. So its not possible to find a nether portal, they can only be generated

Daniel Roberto