Wow, fascists, whether corporate or colonial or otherwise nationalistic really know no boundaries.
In our case we had a guy in the group who claimed to be from a specific place I happen to know very well since I have friends from there and the dialect of the guy just didn’t match his story. This is something very specific to Arabic dialects though where every region, every city has their own dialect. It was very awkward to tell this to the others, because it just ruins the trust in the group.
That's why it’s often better to give menial tasks like putting up posters or painting banners to suspected infiltrators rather than put the accusation out there.
In general I’d recommend having several layers of communication and only let those in which seem trustworthy. If they start having sexual relationships with group members and become leaders of the group like in the article, well… No idea what to do in such a case, this is just crazy.