Alcoholism and custody

In custody issues, what kind of evidence is needed to prove a parent is an alcoholic and not trustworthy of caring for the children? I understand that I can ask for supervised visitation, etc, but how do I actually prove the extent of the drunkeness? This person will spiral downward after separation, without a doubt.

You will need to assert which instances of past behavior show that this person is not capable of caring for the children.