Great question! The most accurate way to know if your gut flora is balanced would be through a comprehensive digestive stool analysis. Most of us don't need to go to that length if we're not experiencing other problematic gut issue like H-Pylori or parasitic symptoms. (Extreme pain ain, bloat, constipation/diarrhea)
Based on your bowel movements and digestion, you should be able to tell where you are and if you're consuming the diet practices I recommend, you'll be naturally adding beneficial microbes to your body.
Elizabeth Lipski, author of Digestive Wellness recommends innoculationg yourself frequently with probiotics just based on every day stressors.
Of course, we have more bacteria in our body than we have cells so consuming lacto-fermented foods such as Kimchi, Suarkraut, Kefir, Yogurts, Kombucha will naturally encourage a variety of forms of bacteria.
I always like to recommend books for great reference based on what I learned in school so with that, I will recommend Probiotics Revolution by Gary Huffnagle as well, Digestive Wellness by Elizabeth Lipski. Both are excellent reads.