Tacking on here, 2009-2013 in Korea as intel guy.
Between a few airborne platforms, conventional ground intercept, and satellite stuff, we have continuous and deep coverage of pretty much everything NK does. In my career, haven't seen such allocation of ISR assets to that degree except, well, Afghanistan, which is a a combat zone.
We have about 30k on the peninsula. IIRC a third are combat elements, the infantry and armor guys towards the frontline. The rest are support or logistics elements, eg several squadrons of F-16, A-10, Longbows, etc.
That 30k? Just a drop in the water compared to what our allies bring. South Korea has hundreds of thousands of troops. While in a direct wikipedia comparison they seem outnumbered by the North (people like to say North Korea has "10 million people", which is misleading and will be explained), they have one of the largest militaries on the planet, and with that, an awesome balance of quality vs quantity. If we are talking mechanized forces (in laymens' terms, mobile warfare), NK only has a slight advantage in standard infantry numbers with APCs/IFVs/trucks available to take undergo offensive or maneuver operations. We can count "10 million soldiers" but the problem is most of those are paper reservists who will not be riding around the frontline, they will be given a PPsh and told to shoot at anybody who comes near their village. If we're going to mash paper numbers together and call it "accurate reading of how war plays out", we might as well point out that SK can mobilize nearly twice as many healthy reservists as NK can. To put it more simply, the number of SK divisions at the frontal area roughly matches the number of NK divisions that will they will be able to put on something with wheels/tracks and send offensively--not a good thing when you are at a technological disadvantage in tanks and the traditional military mantra is that you should never go up against defenders with less than a 3-to-1 advantage (not always true, but good starting point).
The NK artillery situation is pretty saturated, but tends to be exaggerated by Western media. I can get the studies on request, but basically, the whole idea about "levelling Seoul" or NK having some apocalyptic artillery capacity is very misleading. It fails to account for US/ROK counterbattery options, miserable performance of the general 70s-era artillery NK possesses, and limitations of keeping your artilery on call when the battlefield will be shifting 10-20km every hour. However, to be fair to NK, they do possess a credible biological/chemical warfare capacity... this is far more threatening than their conventional or present nuclear capabilites, and will likely be the biggest cause of both civilian deaths and hampering ROK/US military operations.
Enough facts, its anecdote time: when NK shelled the SK island in 2010, we were literally one NK shell away from invading them. Not to leak anything but I saw the ROK operations plans, and it was pretty clear President Lee was going to respond to any further NK provocation with an end to the regime during those tensions (with the full blessing/support of Obama--we had several assets on standby for combat). The funny part to me is that if you asked the average South Korean on the street, or English teacher or whatever at the time, they would tell you that "the attack is a little shocking but nothing special is happening". They had almost 0 clue their government was an inch away from going in on NK.
For that matter, there were several months where NK looked internally to be preparing for war beyond their public statements. There are several tells that separate the bluster from "oh shit they are actually mad". Its the mundane stuff being shuffled to wartime positions that give it away: blood supplies and medical units, food and fuel stocks, etc...