Hi and welcome to the board
Swiss Miss is a Con Agra company, and they will list any gluten on the label. Nestle will, also. If you have other brands you are wondering about, you could list those and hopefully someone will know--or you can always call the company and ask them.
Never assume that all foods in a catagory (such as hot cocoas) are all gluten-free--each one has to be checked.
It's a little daunting at first, but I promise it will get much easier with a some time.