I know its tough to really say if this is an accurate test, but I'll tell you my story.
I did the spit test and it definitely came back positive for yeast over-growth, had the legs, the water got real murky the whole nine.
I was a bit skeptical at that point just like you, so found a naturopath and did a test through a lab for yeast over-growth and it came back over the charts positive.
I immediately started a strict strict anti-candida diet and have seen a huge improvement in all my symptoms, I'm about to hit 3 months on the diet this week.
Your best bet is to look at all your symptoms, although no two people who are dealing with a yeast over-growth have the exact same symptoms, you can be sure that if you have a handful of them, and you have been on anti-biotics (all of us), had/have a sugary or high carb diet and/or stress that your a prime candidate.
The candida diet is a great place to start anyways as it will help you get healthy and build up your immune system.