It's more what I hear than what I see. When they advance from just panting to "noisy" panting, they're getting closer to heat stroke. When they start making long, drawn out calls (they almost sound like they're shouting "....Laaaawwwd") then they're actually getting closer to death. In my experience, if they're making that noise when it's still heating up outside, I've got to get them cooled down quickly or they could be dead inside an hour.
Additionally, disorientation is a clear indicator of pending doom. One of my girls couldn't figure out how to get into her pen in spite of living there for nearly three years. Once I cooler her down she was fine again.
I actually have one hen that went to roost tonight while still making more exclamations than I would like, but every time I tried to remove her from the perch to cool her down she became so frantic I was afraid she'd have a stroke or something, so I left her alone. I'm really hoping I don't wake up in the morning to find her dead. She's one of my oldest girls from my very first flock, nearly four years old now, and still gives me large eggs 5-6 days per week.
I was afraid this might be your answer. One of my girls was making similar sounds, wasn't sure if she was frightened because it was darker than usual when I closed up.