First step is to look at the packaging. The packaging usually speaks volumes about the quality of what is inside. It usually the case if the company does not put much effort into the package they don’t put much effort into the product.
Second Step is to look at the ingredient list. The more artificial flavors there are the flavor will either taste sour or chemically. If it contains MSG I would avoid it completely.
3rd step is Look at the grocery shelves. Does it look like the package been sitting awhile? Does it look like the company is having a hard time filling the shelves? You can use the mob to your advantage here. If the package has been sitting awhile I would avoid it.
4th step is Ask those around you. Look around you. Is there another customer? Is there a store employee? These individuals have vital information for you. The fellow customer might have already tried the product. The store employee can tell you if the product has sat awhile. The employee might be able to tell you if it is popular item.
5th step is Pick a local friendly store. I myself like Beehive Health Essentials in Spanish Fork, UT. You can ask the local owner what products they suggest. You will get better service. You can also shop at stores like Good Earth that has an excellent return policy. They allow you to return products that have bad texture or taste.
6th and final step is to find review resources. There are plenty of bloggers that do reviews on Gluten Free Products. You could use a resource like “The Gluten Free Consumer.” This is a review magazine that features great gluten free products. You can find them at http://www.gfconsumer.com
I’m sure you can find your own rules. These are some of the ones I use.