I once asked Sayadaw Kumarabhivamsa how do they know if the “peaceful state” is really Nibbāna or not when yogis say they attain it. He told me that they watch the yogi for 2 years and do different things and tests with them. He said that in general, those who have pariyatti experience (dhammacariya degree or alaṅkāra degree, etc) usually have it when they say they have it (meaning they pass all the test from 2 years etc.).
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