When choosing healthy foods one guide you can follow is the list of ingredients. Over-processed foods have huge lists of ingredients, many of which are chemicals. Not all of these chemicals are unhealthy, but simple nature based foods are always a safe bet.
In many processed foods, several ingredients are long names of chemical compounds that most people can't even begin to pronounce.
One of the reasons I prefer green tea is that it is the least processed out of all types of teas. In fact, the list of ingredients on the green tea I make is simply, "green tea."
The order in which ingredients are listed on the packaging of food goes from the ingredient there is the most of, to the ingredient there is the least of. So if you see an ingredient that you are trying to avoid at the top of the list, you may want to try something else.
The most important part of this is just being aware of what you are eating. Many people eat foods without even glancing at the list of ingredients. Being aware of what exactly you are eating is a great first step on the way to a healthy nutritious diet that will be an asset to you in your life as a martial artist and as a human.