If your birds have access to dirt on a regular basis, you might worm them once a year. The most common worm is the round worm which is in all dirt. These worms live in the intestinal tract and will live off the food your birds consume. So one symptom is very thin birds. You can tell a bird is thin by feeling that breast bone. There should be meat around it and not all sunken in. Another good symptom is diarrhea. Diarrhea is common in a bird with worms. Another symptom is seeing a bird consume huge amounts of food and still be thin. Eventually however the bird gets weaker and sicker. The worms impact the intestines completely, the crop doesn't empty and of course the birds starves to death.
My birds have contact with the ground as well so I worm them once a year. Being in the UK you might use Flubenvet Poultry Wormer. It kills all worms, their eggs and tape worms as well. Here is a link I dug up for you for an example....
http://www.viovet.co.uk/Flubenvet_W...oa_network=g&gclid=CLHqjL3J8cMCFQozaQodD7YAuw