I have been thinking about asking her if she has had an "affair" and see how she justifys the answer or responds in general, my reasoning would be to see if she tells me the truth.
She'll probably respond with "were seperated and my business is my business" my response would be "well hon, were still legally married and if I need to make a decision about if we are going to be continually married any longer then it is my business to know isn't it?, I would tell you the truth If I did which I have not"
OR
Maybe I should take her to a New Years Party and get a room and see how she reacts if we get close enough to be intimate.