Not necessarily. Certainly some horses with parasite problems can be identified with poor hair coats, but it is possible for your horse to have parasites and show no exterior signs. Most horses are likely parasitized to some degree. The only way to determine conclusively that your horse has an unhealthy level of parasites, is to have a fecal egg count performed.
It is important to note that as the number of parasites increase, the horse‚Äôs ability to use nutrients can be affected, and there can be damage to internal organs.