There are six main types of worms, that have different effects and symptoms. These include diarrhoea, pale yolks, sagging wings, weight loss, gasping, shaking of the head, stretching of the neck and your bird looking hunched. All worms and parasites can cause death in extreme circumstances.
If you can see any symptoms in your birds then get worming! To keep worms and parasites at bay make sure that the ground they are on is clean and the grass is as short as possible as it harbours less worms and parasites.