Of course the GMs try to get proof. Why do you think people get banned for illegal activities involving the server? (Such as botting or 'hacks')
Without getting extremely technical it's hard to explain how the GMs would detect a packet generator.
Pretty much there is a certain way you are supposed to communicate with the server. If things aren't done exactly that way it will flag it for inspection. Like no one is able to spam 1 billion arrows in 2 seconds. So the server flags it, the GMs do a little more investigation, and find the person was doing something illegal, then ban them.
Any known hacks have been patched by the GM team. That doesn't mean new ones won't come out, but if they do the GMs will just patch those too. That is why I said I haven't heard of any hacks that will work.
Anything you aren't supposed to be doing can be detected by the GMs. That's why they are there. The users don't need to go out hacker hunting because the GMs have it well in hand. Now if you do see someone doing something suspicious you could notify the GMs but they probably already know about it.
When they ask for proof they are probably just trying to weed out false alarms and bogus accusations.
We're not talking about packet generators here. Things like WPA are easily detectable since they're a 3rd party application running along the actual client, there are numerous ways to detect that. But then again there's thing called "sight hacking" which requires players to edit few lines
clientside so that they see hidden people. Also you can modify client to display the damage in WoE there are tons and tons of things you can do to make your playing easier without any actual info sent to the server.
There is no such thing as a secure server, no matter how hard you try to make things secure there is always a way to get around it if you're smart enough. There are few extremely good programs around there to stop bots and packet duplicators but I'm quite sure even tyhey aren't 100% secure.
As for detecting them, it doesn't work they way you say it does. What a packet duplicator like WPA does is that it captures one packet, let's say you pick up a card. Then it sends that packet to the server again, fooling it so that you picked another card and another card etc etc. Everything follows standard server protocols and communication standars, "flagging" some strange packet data doesn't happen if there is no strange packet data. The only way GM's could detect this is through item logs, I mean someone picking up 500 doppel cards in a row is a bit suspicious right?
There is no automattc detection, there are programs which prevent the usage of 3rd party programs such as WPA from running. But there is no actual way to automatically detect packet duping server side.