GC,
I'll add to the thread here.
First thing I look at is the exterior - are the rubbery tips on the hooves(eponychium) undisturbed? If so, the calf never walked - they get tattered pretty quickly once one gets up and scuffling around.
Any yellow-green staining of the haircoat? If you get into a fetal distress situation, they will defecate inside the uterus, and the meconium (first feces) will stain the hair.
Internal umbilical arteries - inside the abdomen, running on either side of the urinary bladder - will have evidence of hemorrhage in their wall if the calf had an active heartbeat at the time the umbilical cord became compressed as it passed through the birth canal.
Lungs - if dark red, and sink when placed in water, the calf never breathed.
Now, as to calves 'suffocating due to the sack over their head'... I've never seen one; foals, yes, but horses have a different type of placentation.
Every calf I've ever seen born, or helped deliver was well out of the 'sack', by the time it's head cleared the vulva.
Not saying it can't happen, but it'd be pretty doggone rare.
Have seen a few born alive, then stepped on. Usually they have some bruising or a fractured liver or kidney - if you look inside.