Transformer is one of the crucial equipments in electric power network. Moisture in paper insulation acts as a catalyst in transformer insulation aging. Experience has shown that humidity will affect electrical strength of paper and pressboard as well as other insulation system materials by reducing degree of polymerization and accelerated aging. Thus, it is essential to determine the extent of moisture content within insulation system before exciting the transformer and also during normal operation. In this regard, chemical tests on oil have been carried out for many years. Also, electrical tests for assessing the transformer condition have been developed and improved in recent years while most introduced methods aim to extract moisture from transformer insulation system. In this study, the results of practically applicable methods in oil sampling (KFT and capacitance sensors), paper sampling (Dean-Stark method) and also dielectric response (RVM, PDC, FDS) are examined in determining insulation system humidity. Test results through various methods are compared to analyze the sensitivity of different approaches. Crucial practical recommendations as well as advantages and disadvantages of each method are discussed. At last, to recommend an accurate and reliable method for practical implementations, practical tests have been performed simultaneously on three distribution transformers and the results of them are discussed.