Help define "cohabitation"

[groggnap]

My ex and I got divorced in July 2005 and I pay her alimony. Our divorce documents say I can quit paying if she remarries or cohabitates with someone. I believe her new boyfriend is living with her (about a month or so) since every timeI go to pick up my son or drop off a check, he is there. I believe he lives with her now but can't prove anything. What is the best way to determine if she is cohabitating and exactly what defines cohabitation? Is there a set time period he would live with her? What if it's only 4 or 5 nights a week? Just curious.