Being severely infected is not that easy to tell, but you also need to look at your symptoms. There are many symptoms which are common for candida sufferers and if you are suffering from them then you may have a candida yeast infection.
If you have chronic yeast infections or jock it, athlete's foot, thrush, etc it's going to be far easier to know that you are infected as the overgrowth got so severe that it came on the outside of your body.
In my opinion one of the best ways of knowing if you are infected is to go on a candida diet and see how well you feel while on the diet. Do you see your health recovering? Do you feel more energy, or do you have die-off while you are on the diet? Those things are a great indictator that you may have a candida infestation.
I would also consider getting tested at a laboratory for candida.