I think its funny that many of us can instantly recognize a screamer seconds before it happens.
Here's some of the signs:

1. The initial volume is a little low.
2. There is an obstacle in a flash game that seems like it would take a couple seconds to do.
3. Things are a little too calm.
4. The title of the movie looks too good to be true.
5. The title/icon of the flash game/movie looks promising but the file size is <50.0 KB.
6. If its a YouTube video it is <20 Seconds and half way through the movie it is still doing one monotonous singular activity.

Sometimes you will exclaim out loud "It's a screamer" and scroll down/lower your speakers right before the scream.