You can involve a Vet if you like, but it is just easier and more practical to assume that young horses have parasites (particularly roundworms) and deworm every 6 to 8 weeks. A horse from 3 months to 3 years of age is the most prone to parasites, particularly round worms). We have found Anthelcide and Safe Guard to be more effective against round worms than Ivermectin. We use Ivermectin at least twice every winter and early spring and use the previously mentioned ones other times for young horses.
We have done several postmortem exams on horses from 4 months of age up to 18 months of age that died from round worm loads. I personally filled a 5 gallon bucket (NOT before running out of worms) and I just quit. This was when a good friend found a dead 6 month old foal. It was the second foal they had found dead that year -- both sired by one of my stallions. I had repeatedly recommended that they should start deworming their foals earlier. After they lost a second slick, fat baby, I went to their ranch and did a postmortem on the foal and filled a 5 gallon bucket with round worms, many that were more than a foot long and as big around as my finger and looked like white snakes.
People often associate heavy worm loads with poor, thin, pot-bellied scruffy looking horses that have been neglected and poorly cared for. The truth is that I have seen more horses that died from their parasite loads that were slick, fat and otherwise, well cared for. The young horses have died from round worms and the older horses died from strongyles and complications like colic. Almost all of these young horses were slick and fat. If you feed the young horse well, you are also feeding the worms well and they get HUGE.
Once a new horse has been initially dewormed, then, a fecal exam is in order although horses can have a moderate worm load and not be shedding eggs when they are checked. With horses under 3 or 4 years of age, it is just most effective to deworm them regularly. They probably have worms, especially if they are on a property that has had many horses on it over the years. It is not necessary (or even advisable) to use heavy duty deworming plans (like Power Packs) on young horses. Round worms kill easily and and are going to come back no matter what you do. Just treat for them.