South Korean missile experts have concluded that physical evidence indicates North Korea has not developed any new ballistic missile technology, or even manufactured many new missile parts since 2012. The South Koreans are pretty certain of this because since 2012 they have been able to recover components of North Korea multi-stage ballistic missiles and examine them. These missiles must be tested far at sea (outside heavily guarded North Koreans coastal waters) and by 2012 South Korea had developed technology and techniques to retrieve a lot of these components and have South Korean and foreign experts closely examine them. Initially South Korean engineers and scientists concluded that most of the components appear to have been made in North Korea. The longest range rockets were based on much older (1960s and 70s) technology and the design of the rocket engine was almost identical to one built in Iran. Many of the imported components of the missile were items that are not covered by sanctions, as they have many other industrial uses. The Iranian connection was long known as was North Korean access to older Russian rocket technology.
Tests of three stage ballistic missiles (that can be used to reach North America or put a satellite into orbit) provide the most recoverable parts. South Korea recovers many components of the large rockets because the first two stages fall back to earth in the ocean. South Korean engineers initially reported that the construction of the missile components retrieved appeared to be sloppy and there were some foreign components in the rocket. Further examination concluded that the North Korean components and construction techniques were crude but effective, if not as reliable and efficient as Western or Russian designs.