For instance, this is a perfect one to bring up in the book of Deuteronomy:
18 If someone has a stubborn and rebellious son who does not obey his father and mother and will not listen to them when they discipline him, 19 his father and mother shall take hold of him and bring him to the elders at the gate of his town. 20 They shall say to the elders, “This son of ours is stubborn and rebellious. He will not obey us. He is a glutton and a drunkard.” 21 Then all the men of his town are to stone him to death. You must purge the evil from among you. All Israel will hear of it and be afraid.You don't even need to start arguing with them. Just bring it up and watch how they try to rationalize it. There's wide variety of "explanations" that will come up.
There are a couple of answers that would elicit a bit of retorting. If this is the type of Christian that argues that we cannot apply this law nowadays because the elders being referred to is talking about those of Israel, and that system is not in place anylonger, just retort that that's not the point. The point is whether he or she thinks that the death penalty here is justified. Either they have to agree with the death penalty in a case like this, or they have to oppose it; many such Christians will have to admit to agreeing with the death penalty (followed by a bunch of whitewashing a femtosecond later), and that's the point.
Another explanation is that this particular law is not in effect anymore because of Jesus or something. This is just dodging the actual issue. Regardless of whether it is in effect after Jesus, it was certainly in effect before, and therefore the death penalty for such a "crime" was good and right and in accordance to God's will. Ask if they agree with this.
Some will not know what to say, and will often say something to the effect that they don't really understand it, but surely there's some higher reason for this that only God knows. You don't even have to answer anything to this.