The first sign the presently sick mare showed? Her head carriage was not quite right. She naturally has quite a high head/neck carriage and today was holding her head at a 'normal' height, for another horse. Not for her. That progressed to laying down a lot, but not looking unhappy - just looking like she was happily sunbathing. I checked her on my way past [because I know she colicked yesterday, so everyone and his dog who knows about it has been checking on her] and then 20 minutes later found myself helping to try to keep her from throwing herself on the ground. The vet was thankfully called early in the piece and the horse is now at the hospital being treated but it's the worst colic I have ever seen, so while the vet is hopeful, the owners are worried sick.
Please know your horse, know what's normal, so you can pick up on the subtle signs it may show that something isn't right! The sooner you can call the vet, the better the outcome for everyone involved. This is a very very sick horse that, to someone who doesn't know her, looked perfectly healthy... up until the point where she started throwing herself to the ground. Violently.