How did you____that the man was a thief and was trying to steal my money.

答案是B
四个选项都可以表示：看.
句子意思为：你如何注意到,那个人是个小偷,并且试图偷我的钱.
see,最常见的,但表示看到,如,I saw your mother yesterday.
watch,常用于看电视,如watch TV.
observe.表示观察,常用语 科学观察,如观察天体,observe the moon.还可表示庆祝,observe Children's Day.
B.notice 表示注意到,觉察到,最符合句义.