Here is what experts in the article had to say about detecting such infiltrations:
A South Korean military official, speaking on customary condition of anonymity, said that while South Korean troops aggressively monitor the border with advanced surveillance equipment and their naked eyes, their ability to monitor some portions of the DMZ is limited.
North and South Korean troops man outposts along the Military Demarcation Line, often in forested areas.
“It’s very difficult to keep watch there for 24 hours a day, especially if it’s rainy or foggy,” said Kim Seongmin, a defector and former North Korean military officer who now heads Free North Korea Radio. “If North Korean troops want to violate the armistice agreement, they can do it there,” he said.
An Chanil, who was stationed at the DMZ as a platoon sergeant in the North Korean army, agreed.
“Nobody can see everywhere,” said An, who defected in 1979 and is now president of the World Institute for Korea Studies, a Seoul-based think tank.
He said ordinary North Korean soldiers would have difficulty crossing the demarcation line, though specially trained reconnaissance soldiers could do so with ease.
“The DMZ is their home ground,” he said. [Stars & Stripes]
You can read more at the link, but this is why any North Korean soldiers caught violating the armistice should be shot on site. In the past warning shots were fired when they were caught crossing the DMZ. It is now time to start attriting their operatives who are specially trained for these missions. That will be a deterrent to continuing to launch such attacks.