Citizens are methodical when it comes to war. Between their tendency towards what we would call vendettas and the price of mercenaries, there is a cultural emphasis on making sure there is nothing left to carry out reprisals. Limited supplies and fear of human numbers also encourage them to take assured steps. (You'd be scared if stuck on a foreign world, even if the populace didn't have things like tanks and reanimates - right?)
If you piss them off - you will know it. Fortunately, it takes a while to get that far.
For the most part, they aren't looking for conflict - since doing so requires such extreme measures, and most of them probably don't have the supplies to carry it out for long. At least we don't think they do.
Patrols will be seen around their borders, but they tend to announce their presence or leave signs - deterrence only works if you know its there. In some places - usually desert like ones where there are few people, it will be just citizens in flack vests with their little rocket rifles, and panzerfaust like anti-tank rockets. They don't like facing humans without their machines, however. Air patrols by helicopters, roving beetle like armored cars, or perhaps most likely - their signature demon armors are another common way to enforce territory.
Speaking of which - laser towers. Those big lotus like things that use phased array emitters to steer energy beams - not always strictly their territory. Obviously, the aliens have to be around to set them up and keep them charged. But you can't always trace a line between them and say one side is PC territory, and the other anarchy zone. Sometimes they keep the tower's closer to the settlements for protection but claim a wider area, and others they put the things way out to protect strategic points. Sometimes human settlements can be included in their territory - its nice to have traders, translators, potential slaves, or whatever other use they find for humans at times. (So far as I know, they don't find us delicious.) Also - Citizens aren't dumb and will take up active defense if you prepare to make trouble on their borders - they will not just sit patently on the other side of the line while you prepare for war.
Where was I? Right.
After patrols, the most likely sight will be wrecking crews. Citizens know about denying sanctuary and supplies to their enemy and often go about demolishing old towns to make sure they don't end up with refugees, bandits, or worse on their borders. Usually these teams will give advance warning to evacuate, or even negotiate a scheduled for moving the populace. Usually. At the appointed time, teams will move in with explosives, backed by Succubus units (the ones with missiles - incubus is just guns) to collapse anything that tries to resit or attack the sappers. After that, they tend to either sift through the rubble for any valuable materials - or just cover it with dirt to make a small hill. That is a sure way to tell the difference between a town depopulated by reanimates or hit by raiders that found a cache of missiles - the aliens are careful to not leave ruins, reanimates leave most intact, raters just leave junk.
When you do find an actual citizen military unit, it tends to be big, and some multiple of five. Can't tell you why that is the magic number, or the names of their units, but its always something like five, fifty, or two hundred fifty - though that last number is pretty much unheard of these days. Just think in terms of a normal human platoon, company, and so on. Often some portion of these numbers are mercenaries, and rarely that means human mercenaries. Two to one you're going to faced trained high level warriors rather than conscripts. A bit of selection pressure means that those who made it through the Event learned how to deal with tanks, so even a few of those aren't going to be much help, and really - who has "a few"tanks just lying around these days?
Basically - don't tick off the aliens, OK? Don't you have enough problems already?