Take a look at the label - if a plant-based food source is not listed anywhere, and a chemical name appears in the ingredient list, it is synthetic. For example, ascorbic acid is an isolated component of Vitamin C, and is not the full-spectrum version of Vitamin C found in food sources such as amla berries, kale, broccoli, or strawberries.
Even if the product is food-based, some products “boost” the formulas with synthetic nutrients, or add sugar and yeast-based ingredients to the mix.
Learn how to read supplement labels here.