They can be observed without infiltration, but as MestaMachine pointed out, even that could be very dangerous for the average civilian.
We have here a sort of network we lovingly call the Mountain Telegraph. Most of us are pretty secluded and can’t see our nearest neighbor, BUT, if anything moves funny, any stranger comes off the main road, moves anywhere near or on any of our properties, all the rest of us know it within a matter of minutes and whoa nellie bar the door because you are SOOO busted.
I think maybe a system like that to monitor the comings and goings, what groceries or other things they buy for changes in habits, listening for weapons fire or explosions, (because they train with these things and have been heard,) and whatever little details you can grab overhearing conversations, stray words in shops or stores. Sometimes gossip is gossip, but sometimes, you can make a fault into a virtue. This is one way of doing just that.
Bear in mind that these guys are training for guerilla warfare. They practice stealth. They might have caches in strategic places.
ALSO bear in mind that we aren’t the ONLY ones who can disappear. That’s a fact.