Candida is not only natural to the digestive system, but is essential to life. In times of famine, the good flora could feed on the Candida yeast to stay alive until there was new food to digest.

What is not natural is Systemic Candida, where the Candida has escaped from the gut and is colonizing other organs. This is a serious and very dangerous situation. Most diagnosis of Systemic Candida comes from symptoms. There is a list here that is very helpful: http://Lufe.info/symptoms If you have these symptoms, then a test is not needed, you should consider treating the Systemic Candida. The doctor can prescribe Diflucan, or you can get Lufenuron over the counter.

if you mean a stool test, it can be used, but it is known to provide false negatives, the candida antibodies blood test is probably the most reliable indicator when testing for candida infection:
http://www.candidacurecenter.com/candida-explained/testing-for-candida/