Do keep in mind, that the word Colic means "a pain in the gut". There are many different kinds of colic that a horse can develop.. the most common being:
Impaction Colic: This is when there is something actually stuck in the gastrointestinal system. Usually a big block of food that wont progress any further down the track. This causes food to back up in the system. This is typically a "call your vet" situation. Put your ear to the horse's belly.. if you don't hear any bubbling or moving around after a few minutes.. this is probably the type of colic you are dealing with.
Gas Colic: When the horse gets very bad gas, sometimes from a mixture of food they have eaten (lots of grain and lots of water on a very hot day will actually make "beer" in your horse! It firments and gets really bubbly.. my App had this happen this summer), or the amount of food and water they have followed by extreme excersize. This can be handled with the correct treatment and sometimes does not involve a vet coming out (though its better to be safe than sorry!) Put your ear to the horse's belly.. if you hear a lot of gurgling and bubbling, then that is probably what it is.
Twisted Gut: This is the worst possible type of colic.. from my experience, this typically results from a horse with the other types of colic that goes ignored and is left to its own devices. Rolling. When a horse has a stomach cramp, they can't hold a warm rag over it or treat it.. they roll around hoping that they can work the cramp out. This is when bowels get twisted. Only a vet can tel you if your horse has twisted its gut.. and this needs surgery or else the horse will need to be put down =/
As barnrat said, if you see signs of colic, walk your horse CONSTANTLY until the signs have been completely gone for about a half an hour. Do NOT let them roll or lay down. Walking them out will help stretch their muscles and body a little bit, helping them better get rid of the gas and hopefully unblock the impaction.. if nothing else, this will help ensure your horse wont twist its gut. And as savethepitbulls said, Sometimes you can catch colic early on when you see your horse acting odd or not eating right. Usually those signs can mean a large array of things, but some people know their horse -very- well, and will notice the change in behavior, and go in to investigate whats wrong. =)
Rockymountain, does this mare have any foot or leg problems? If she is constantly stretched and laying down, it sounds to me like her feet are really hurting her. Horses stretch waaaay out to distribute their weight off of their soles and onto their toes/heels in a way to avoid the pain from their own weight. If it was colic, she would have been in oodles of trouble by now :) Colic is not a chronic illness =) It comes, does its thing, and gets treated and leaves =)