kareninthesun,
As you research breeds (there is a breed section here on BYC) and pay attention to the origins of the chickens, you will definitely get some hints. Also there are notations that will say something like 'continues to lay in winter'--- Thus a cold hardy chicken.
in general -- Mediterranean breeds like White Leghorn -- are good in heat, and the very heavily feathered extra large birds (think wooley mamoth) with smaller combs that will resist frostbite, are better cold climates. Feathered legs are anther example of varieties of birds that are cold resistant, thus heat susceptable.
It is a really excellent question in this horrendous heat wave. Hope this helps a little and if others have different views they will put thier thoughts in the mix.