If the saliva in a glass of water sinks to the bottom, forming wavy threads, does it mean that you have an overgrowth of yeast?

I suppose you are referring to the so-called 'home Candida test', which involves spitting into a glass and observing the behaviour of saliva in the water. Such a test is not reliable and serves no purpose. Candida albicans occurs in the oral cavity of most people and only in some (especially with impaired immunity) can cause ringworm (candidiasis), which will manifest itself in a cheesy bloom inside the mouth or ailments such as burning, pain and redness. A recognized diagnostic method in this case is a swab and culture.