It is quite another thing, if a man has lived for some time with the girl, had a total way of life, interests and hobbies, then painlessly and quickly separated, left without looking back. Such relationships do not last long and, as a rule, do not leave in the soul of man is no trace. Men are generally alien to many experiences - met parted, went further. Therefore, if he even thinks of ex-girlfriend, it's nothing you will not be.
If your guy falls now and then intersect with the ex-girlfriend (joint children, live near, work together), then it is necessary to be alert. Watch for him, his reaction to talk about ex-girlfriend, gently ask, as it relates to the fact that they have often seen. If a person does not care, then it would be an appropriate response - indifference.
Gather information on ex-girlfriend useful - you figure out what type of women he was attracted, get a chance to know him better (habits, character traits). Often former passions are eager to share information, and sometimes even give advice or caution - in any case, their reaction to your inquiries and the information obtained can tell a lot.