It is an important approach to preventing nuclear proliferation that comprehensive nuclear test ban treaty is reached. How to judge whether a nuclear test has happened is a key point. In this paper we investigate a method to distinguish between a nuclear test and leakage from civilian nuclear reactor by measuring inert gas xenon nuclides 133mXe, 133Xe, 135Xe, and 131mXe. The distinction between the nuclear test and the reactor leakage is found by analyzing the variation of nuclide quantity with time in a complex decay chain. Two suspicious events in North Korea are measured and analyzed. In order to verify the theoretical results, an experiment of imitating plutonium irradiated by thermal neutron is designed.