There are a number of companies that will always clearly disclose any gluten source in the foods they make. If you don't see "wheat, rye, barley, barley malt, oats" on the labels, its not there, or hidden in "flavors, starches, etc." http://www.glutenfre...lists/index.htm This makes shopping MUCH easier.
Further, US manufacturers are only required to label the top 8 allergens at this time and barley and rye are not in the top 8.
Rye is almost never found in ordinary food. It only turns up in things like rye bread (and variants thereof such as pumpernickel).
Barley is the concern. If the manufacturer is not one of the ones in the link above, then there is a chance that barley is hiding in natural flavors. It is a small chance, since barley malt is expensive and so is usually identified (watch for it in chocolate).