In the alpacas' natural environment, it is virtually impossible for them to over eat and get fat. Unfortunately, that is not the case in North America. As well as the obvious risk that this poses to health, obesity also leads to lower reproductive capability, and birthing problems.
At the other extreme, a sudden drop in weight should be a cause for concern as well. Alpacas are very stoic animals - they hide illness extremely well. Quite often, weight loss is the only indicator of underlying health problems that call for your immediate attention.
Too prevent your animals from slowly creeping up in weight or for detecting a sudden drop in weight, it is extremely important to weigh them routinely - on a monthly basis at the very least (crias more often). For this you will need a digital small-animal livestock scale. This is one of the first pieces of equipment we purchased and has been of of the best investments for our alpaca's well-being.
During the weighing process, you should also perform body score checks to assess their overall condition. This consists of feeling the animal near the spine, between the withers and the hips, and assessing the amount of fat.
The score is reported on a scale of either 1 to 5, or 1 to 10. In either case, the lower score represents being seriously underweight, and the higher one, seriously obese. You obviously want to be in the middle, at either 3 or 5, depending on the scale.
Routinely assessing both weight and body score and keeping accurate records can help give you an accurate picture of your alpaca's health and well-being. It should be a part of your regular herd management practices. If they could, your alpacas will thank you for it.