A (probably apocryphal) story which I heard when spending a few weeks at a facility deemed of the greatest interest to spooks a couple of decades ago was that the security services tended to identify infiltrators by questioning the bar maid at the local pub.

Apparently, regulations required the agents to only have small beers, presumably to avoid indiscretions while inebriated.

Only problem was, none of the locals ever ordered small beers, as the general practice was to get wasted enough to forget what a godforsaken place you'd wound up in, if only for an evening.