The worst screaming I can ever recall was a girl who was petrified of spiders. (so scared of spiders she once jumoped for a moving car to avoid one

). A group of us were in a old outback cabin powered by petrol generator, at night, when she saw a huge huntsman spider on the ceiling. The generator chose that moment to go out. In the pitch blackness, her screams were so loud they were almost visible!
It only took about 5 minutes to get the generator back on again, but she screamed non-stop, hysterically for the whole time. She was a wreck for a few days after that, both physically and emotionally.
It was scary, in a way, to hear her scream like that, even though we knew the spider was harmless (BIG, but harmless).
If they are screaming out of real fear, you'll be able to tell, believe me.
Cheers
Mick