摸透 is a verb-complement compound (动补结构) meaning 'to have a thorough understanding of' someone's character, intentions, or a situation. The complement 透 adds the sense of 'completely / through and through'. It can be split with 得 (摸得透, 'can figure out fully') or 不 (摸不透, 'cannot figure out') to express possibility — a common pattern for resultative complements. It's most often used with 脾气, 心思, or 情况.