What does 我就立刻知道他准是对儿媳干了偷鸡摸狗的勾当 mean? (from 活着, p.3)
我就立刻知道他准是对儿媳干了偷鸡摸狗的勾当. 


准 means surely;definitely.

是 is used as an adverb:


  （重读， 表示坚决肯定， 含有“的确、实在”的意思）


E.g. 他是不知道 // He certainly doesn't know.

对 is a preposition, meaning to. E.g. 你对我做了什么 // What have you done to me.

干 means 做(do). 

Over all, the sentence roughly means: 


   I immediately knew he surely had done some stealthy, shady business to his daughter-in-law. 


P.S. As you suspect, the sentence might invoke some sexual affairs.