As with medications, manufacturers of vitamins and other nutritional supplements are not subject to the same labeling practices that food manufacturers must adhere to.
However, many bottles and packages will include a list of allergens that are not present in the product. But always look for “gluten” to be listed. If the product simply lists that it contains no “wheat” or “barley,” for example, that’s not a clear enough answer for you. You need to see “gluten.” Call the manufacturer to confirm.