First, try to determine what is a normal amount of manure for this horse.

Look around the run, stall or corral.  Count the number of piles of manure that are passed by your horse in a 24-hour period or in one day. Horses pass large quantities of manure, often many piles in a 24 hour period. If you do not clean your horse's stall yourself, ask the person that does.

Next, assess the consistency of the manure. 

Wearing latex gloves, pick up a fecal ball. Are they hard and dry, soft and playable, or too watery to pick up at all?  

Hard dry fecal balls may be normal for some horses, but could also be indicative of dehydration. Softer manure may be normal for some horses and, in the absence of other signs of illness or abnormalities, should not cause concern.

However, liquid diarrhea should always prompt you to contact your vet. While a horse may pass 1 or 2 piles of very wet manure when they are temporarily stressed, it should not be left to continue without a veterinary evaluation.

Next, assess the manure for texture and general appearance. 

Do you notice a yellow film on the manure? This is usually mucus and can indicate that this particular manure sat in the small colon for a longer period of time, allowing mucus produced by the small colon to coat it. 

Orange grubs in the manure are bot larvae and are often seen after deworming in the fall. Parasites are killed by the medication and are shed in the manure. Other than immediately after worming, parasites in the manure should always be considered abnormal and prompt a call to your vet.

Horses normally have small fibers in their manure. If you notice long pieces of hay (over 1/4" long or 6mm), the horse may not be chewing its feed properly or the hay may be too coarse. Your vet should examine the horse's teeth and/or the hay.

Horse owners are often pleased to see their horse pass a pile or two of manure during or after an episode of colic. While this is a good sign, it should not be given undue emphasis. Taken alone, the passage of manure should not be interpreted as a sign that your horse has fully recovered.

The equine digestive tract is complex and lengthy. The equine small colon alone is between 8-10 feet long. A horse suffering from an impaction or other intestinal accident upstream may be able to pass some manure, without resolution of the underlying condition.

Nevertheless, it is helpful for you to share your observations with your vet, because manure production is one of many considerations that vets use in reaching a diagnosis and recommending treatment.

Depending on the circumstance, it may be helpful to take a picture of the manure and send it to your vet for discussion.