This is a problem in a lot of states. Here in my state, it's a no fault divorce state and it seems to take a true act of destructive behavior to protect our kids. If it's a serious concern to you, I'd consult with a lawyer before you videotape someone or record them, etc. It may not be admissible in court depending on state laws.
You need to know your state's laws and how they protect children. My state, too, also favors joint custody and I wound up staying in a dysfunctional marriage because I didn't know how to protect my son. I can't go back and undo what I did, but I often wish I had known the laws and what I actually could or couldn't do instead of thinking that staying was my only answer.