I found this video recently, and it's a pretty good example of what NOT to do when using dry ice: https://www.youtube.com/watch?v=VZOI13wp8MY
If you scroll through the comments, someone a few comments down pointed out what was wrong with this method and a few warning signs.


Some other things to look out for: Excessive Peeing or pooping(wet, really smelly poop is a sign of fear) while they are still alive, desperately trying to claw their way out, open mouthed breathing, extreme twitching.

Some ways you know you are doing it correctly: Calm rats(they'll try to get out, but not desperately), labored breathing without their mouth open(it's just because the air is becoming thinner, like when you go to the mountains and you breathe heavier), calmly cuddling in one area(the co2 will make them drowsy, and rats tend to sleep on top of each other). They'll pretty much breathe heavily until they they fall asleep and die. A little poop or pee is normal as they are probably slightly freaked out from being in a new area, and their muscles relax when they are dead, so it is all released.