You should focus on exactly what they say: whether the findings justify their conclusions.
Look at each technique/experiment they use to test their hypothesis and decide whether it supports their final claims. This isn't black and white - sometimes a result can hint at a conclusion without actually definitively proving the truth of that conclusion.
If you see a gap in logic, then you should point that out too.
If you think that something wasn't done and it should have been done, then you can tell them that.
I'm assuming Perilloux, Easton and Buss (2012) is an experimental paper. If it isn't then my advice is useless.