Alpacas are pretty hardy and are very easy to keep. However, it is important that they are vaccinated regularly against a few infections. Blue tongue came in to this country quite recently and Cloistridial infections (similar to ones which cows and sheep get ) should be vaccinated against. It's also important to control parasites, either by careful management of the pasture or by vaccination.
Recently some alpaca herds have succombed to TB and it is very important to obseve good hygeine with your alpacas. Keep water bowls and buckets raise above the ground to avoid wild animals drinking from them. Always clear away uneaten food to discourage other animals coming in and feeding. be aware of any badger activity and keep alpacas away from badger runs and latrines if at all possible.
The best way to spot a problem is to get to know yor alpacas so that you can identify if one of them is not behaving as they normally do.