Have you considered faecal egg counts? In England this service is usually offered by vets and usually costs around £15 and involve taking a sample of the faeces and counting the number of worm eggs found within it. Worming is then done depending on the result of these tests, so if a low burden is found then you dont need to worm your horse but if a high burden is found then you can worm your horse depending on the type of worm burden found. It helps to prevent the worms building up a resistance to the current drugs.
Ideally all the horses on your yard especially if they share the same pastures would need to have it done, although it can save money as most of the time your horse dosent need to be wormed anyway. If this idea is not practical, then try to bring your horses worming into the same pattern as the other horses worming, as otherwise your horse will keep the worms alive and then reinfect the other horses and vise versa, making the worms more drug resistant than usual.
Try this link for more information:
Equine Faecal Worm Egg Count
or do a search for faecal egg counts as there is a lot of useful information on the web. Your vet should also have some more information. Another solution would be for your yard to get the equipment and everyone can do the examination themselves as this can be possible with a bit of learning, although the equipment can be quite expensive to start with. Hope that helps anyway!