Children of that age are naive about many things. A trained 'nice cop - nasty cop' setup would easily trip them up. Once they've admitted it verbally (and they will certainly know) getting them to give a demonstration is easy.

Even more unethical method - Isolate them from their parents and say they will never see them again unless they can escape. If they can break free then they have the strength. If they can't then they will show increasing distress and the test can stop. (I wish I hadn't thought of this, it's reminiscent of Dr Mengele)

Children of that age are naive about many things. A trained 'nice cop - nasty cop' setup would easily trip them up. Once they've admitted it verbally (and they will certainly know) getting them to give a demonstration is easy.

Even more unethical method - Isolate them from their parents and say they will never see them again unless they can escape. If they can break free then they have the strength. If they can't then they will show increasing distress and the test can stop. (I wish I hadn't thought of this, it's reminiscent of Dr Mengele)

Find out what they really want for Christmas (other religions and customs may apply) and tempt them with it if they'll demonstrate their abilities.