When it comes to North Korea US Navy tracks their subs with nuclear powered subs. Just in case you haven't known that yet. It was pretty neatly presented in yesterday's article from Chosun Ilbo, a South Korean newspaper which added that South Korean conventional subs don't do that. US Navy subs wait for them just outside of the territorial waters and stand still until the NK sub will move. I don't know if they do that on regular basis or just when there are signs of such thing as missile launch. Then they track them down and follow. The depth of the Sea of Japan makes it a pretty easy task for US Navy as North Korea seriously lacks in the domain of ASW of any kind.
it must be a very silent service, as:
“U.S. Strategic Command systems detected and tracked what we assess was a North Korean submarine missile launch at 3:29 p.m. CDT, August 23, 2016,” said an Aug. 23 U.S. Pacific Command statement. “The launch of a presumed KN-11 submarine-launched ballistic missile occurred off the coast of Sinpo. The missile was tracked over and into the Sea of Japan, approximately 300 miles off the coast of North Korea. The North American Aerospace Defense Command determined the missile launch from North Korea did not pose a threat to North America.” etc. in
North Korean Submarine Missile Launch Could Complicate BMD Strategy