South Korea – North Korea appears all set to detonate an atomic device, but confirming the explosion when it takes place will be virtually impossible for outsiders, specialists said Tuesday.The best indication of a test will be seismic tremors and abnormal radiation in the air, but even that can be masked if North Korea wants to. In all likelihood the first word of the test will come from Pyongyang itself, just as it happened when the country conducted nuclear tests in 2006 and 2009.Last week, North Korea warned a third nuclear test is planned to protest toughened international sanctions meant to punish it for firing a long-range rocket in December. The world sees the launch a ballistic missile test banned by the U.N. while Pyongyang says it is only shot a satellite aboard the rocket into orbit as part of a peaceful space development program.The U.S., South Korea and their allies pressed the North to scrap its nuclear test plans, saying that will only worsen the country’s decades-old international isolation.The threats have placed scientists and experts in South Korea on high alert as any test is likely to aggravate the already high tensions on the divided Korean peninsula.South Korea’s Defense Ministry said Tuesday it believes North Korea has nearly completed nuclear test plans, confirming satellite analysis last week by the U.S.-Korea Institute, a research group at the Johns Hopkins School of Advanced International Studies.