Gapeworms are a nematode worm, which are long, bright red worms that attach themselves to the inside of the canaries trachea.

The symptoms of Gapeworm are regurgitation, coughing, sneezing and discharge from the eyes and nostrils. Infected birds typically gasp for air and breath with extended necks.
Gapeworms produce eggs that are coughed up, swallowed and passed out of the host through the faeces. These eggs or the resulting larvae are then picked up by the other birds during feeding.
Gapeworm can be present in such numbers that they may completely fill the airway, resulting in extreme respiratory difficulty and finally death.

You can guess based on the symptoms or diagnosis can be made by a Vet using a faeces sample.

Effective wormers are available such as “Worm-Enda Plus" from aristopet. Ivermectin, which can be administered using a crop needle by a Vet or can be done by yourself if you have used a crop needle.
You will still need to obtain your Ivermectin from a Vet as the Vet mixes the required strength, and will advise how much to use. Ivermectin is designed for use on sheep and cattle and should not be diluted with water. Two doses should be given 2 weeks apart and the cages should be cleaned to avoid re-infestation. Note that too much Ivermectin will kill your bird.

University of Nebraska – Institute of Agricultural,

Published: James Hart 07/05/05