You know your horse better than anyone else. If a behavior looks unusual, out of the ordinary, weird, troubling or just plain funky, don't hesitate to call your veterinarian.
Every day, give your horse the once-over. Does she look relaxed? Has she eaten? Drank the usual amount? Good.
Take a look around your feet. How's the poop looking? Normal? Loose? Dry? If there is no poop—that's not good. If your horse went all night without producing any manure, it's time to call the vet.
Lay your head on her side. Too noisy? Too quiet?
Walk her around a bit and see how she moves. Colic hurts and you'll notice a difference in her gait if she's feeling pain.
Remember: trust your gut if you think there's a problem!