Once you experience an actual Candida albicans overgrowth in your body. Candida albicans in the intestines can cause a number of dreadful symptoms ranging from debilitating headaches, dizziness, and foggy headedness to leaky gut syndrome which can lead to numerous food allergies. There are a number of lists naming the various symptoms of a Candida overgrowth posted all over the internet, so a list should be fairly easy to locate. There are also online questionnaires which will tell you what your chance is of having a Candida overgrowth. To be honest, these are probably just about as accurate as a lab test since lab tests are not all that dependable.
A healthy diet can never hurt, and it's imperative that you have a good Candida diet when you're fighting a Candida overgrowth.