So, we have to be sure it's very reliable by maybe comparing it with other fairly reliable methods.
If things get complicated, I would choose the King Solomon method. It can be effective at revealing truths if the judge is GOD's servant.
About the judgment of King Solomon, it could be another good method but what if both parents will act as the same?
You mean like the thief faking her emotion and pretending to want the child to be saved?
Assuming that's is what you mean, I will likely get them both to make great sacrifices for the child... probably by keeping the child safely in the middle of lion's den and tell them that whoever reaches the child first owns him/her. I will observe how they try to solve the problem to get to the child.
Or will simply continue to interview them and do certain things to know the truth and how much they care about the child.
By the way, it's GOD that will direct me on what to do and how to do it right. I wouldn't trust my methods and judgments without HIM guiding me.