Acute infections show no real signs other than perhaps a little mucous. Chronic infections in the mouth look like white firm fluff which adheres to the lining of the mouth, and will often bleed if disturbed. It is also very painful for the bird and the bird will look fluffed and depressed. Bacterial and other fungal infections can look visually similar to Candida infections. The only definite way to diagnose this condition is to take a swab of the affected area and examine it under a microscope for the evidence of yeast organisms.
Treatment depends to some extent on the circumstances.
Single pet birds and hand-raised juveniles can be treated with Nystatin drops or amphotericin. Flock treatment is best achieved with ketoconazole in the nectar. The others are available on prescription from your avian veterinarian. As always, prevention is preferable to cure. Optimal hygiene and management are essential.