I dont think its very easy to tell

As you say, esp coerced. Does this count: “I know you don’t want to do this but if you smile and act like you are having fun you can have your iPad back. You do want to be an actress right?”

Or this: “okay little star, pretend like you hate me and I’m hurting you. Let’s see how good you can act”.

It seems to me that even abuse could be subtle enough to be interpreted as the girl wanting it.

I don’t anyone should be judging this except the viewer and only the producer should be investigated if abuse is obvious or suspected. Laws against viewing are too subjective and should be eliminated entirely.