I'm having trouble understanding this:
My translation is:
[With] his finger picking trouser-leg mud, [he] angrily told me that it was his unfilial son, [but] when I next asked him why hit him, he hummed-and-hawed [and] spoke unclearly; I immediately knew he (准是对 ???) [his] daughter-in-law work (?) [in] stealthy, shady business.
Question: What does 我就立刻知道他准是对儿媳干了偷鸡摸狗的勾当 mean?