I dont think you should talk to the ex at all, that just seems like something that could cause alot of fitnah and distrust between people.
If your really curious about how they treat their ex- spouse, ex-fiance, see how they deal with the matter by actions. Like do they backbite about them, tell you faults about them, try and make them look bad? or are they respectful about the situation and try to protect the integrity of the other person. You can learn alot of things about a person when you observe how they act in certain situations.