How can one differentiate between saliva and pus or purulent discharge, especially with a strange taste in the mouth after orthodontic treatment, and what is the ruling on the white sputum that appears in this situation?

Pus is a thick discharge unmixed with blood, while 'sadeed' (serous fluid) is thin wound water; both result from wounds. Saliva, however, is pure. As long as there is no wound in the mouth, what you see is normal, pure saliva. Any doubt about its purity or whether it is pus or 'sadeed' is merely a baseless, obsessive whisper that should be ignored. The altered taste may be an illusion or a result of dental braces.