First of all I highly recommend you read what yeast infection/candida is by visiting this page so that you can learn more about the infection itself.
You know you have candida when the yeast infection symptoms are chronic or out of control. For example let's take two-three symptoms and go over each to understand when it could be yeast and when it's just normal.
1. Bloating/Gas - Everyone gets bloated/gas now and then however if you are experiencing it on daily basis where it's having an impact on your daily activities and your life then you know there is something going on and you better get it checked out or get probiotics like probacto which really helps when it comes to bloating/gas.
2. Itching - Who doesn't itch? We all itch, somewhere now and then but if you are itching non stop which is causing rashes/redness around the area then it's not normal and its a sign of skin yeast infection. In this case you might want to check out some candida diet and take probiotics with it. In men itching is mostly around jock area.
3. Smell - This one is for women out there and every now and then down there can be smelly but if the odor down there is at a level where its having impact on your sex life and other activities because people at job can smell that odor then you have yeast infection of vagina and you need to get on a proper candida diet asap.
I hope this helps answer your question how to know the symptoms have turned it into yeast infection.