Yeah, if they are fine all day and the next morning one is dead... they are dying from suffication as they pile to stay warm.
Best thing to do at night as we deal with this quite often in Ohio, especially around fall, is to tarp the whole pen so it traps in the heat. I tarp off the whole pen to eliminate draft and to trap the heat as 100 2 week old broilers will put off quite a bit of heat.... plenty to stay warm.
I would try this and save the bird in the freezer. If your problem is solved than no need to test the birds as chances are it's just from them getting chilled. Put a booster in their water and cover them up... if they are cold... they will stress.... lose weight... and get sick form a disease that a stress-free chicken is immune to.
Being too cold is more stressfull on the chicken than being too hot. As long as a chicken has water and shade it will not stress near as much as if needed to pile to stay warm.
Good luck.