Let's talk about catching an individual, rather than organised groups.
It's a common sense thing, I tend to spend a lot of time at our local river walking the dog or passing by it on the school run etc - our members are supposed to display a card in their cars, some don't (some people are dog walking etc and nothing to do with the fishing) ... but you usually get an inkling if something isn't quite right.
If I don't know someone, I might take a walk with the dog like any other member of the public might, then pass by them discretely and just ask how the fishing is going ... quite quickly someone legit will usually offer you enough detail so you know they're ok ... if they don't, then perhaps work out a question that will 'out them'.
I ask people if they're on a day ticket (you can't get one unless fishing with a member on our river

). I can then either decide to take further action there and then or simply wander off on my way and call for back up without the 'visitor' being any wiser.
There's a lot of gung-ho talk re what should be done in these situations, the reality is not as simple as might be suggested in the pub conversation when you are in the field, unless you wish to end up wasting a lot of time defending your own legal position for the sake of catching what is most likely a below par angler, who may not have the skills to empty your river/stream/lake anyway. In my experience good anglers are usually keen to keep things legit.
Now bring on the stories which demonstrate that I'm wrong

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