Because they grow in colonies, often along interstate highways, and from late summer through winter feature showy red fruit clusters, most people can recognize a stand of sumac from afar. Smooth Sumac (Rhus glabra) is not the only species with forage-able fruit, but it is the most common species I find locally. Aside from terminal clusters of red berries, notice Smooth Sumac's bloom-covered smooth young twigs and large pinnately compound leaves.
As with most wild foods, timing is important with this small tree. Spring is the time for tasty sumac shoots, and August is prime time for ripe fruit. In the field, a simple test – lick your finger, touch, and taste – will reveal if clusters are worth harvesting.
A few flavorful clusters, when placed into cold water (hot water will leach out bitter tannins), will lend a lemonade-like flavor to the liquid. Experiment with the fruit/water ratio and brew time to achieve a satisfying flavor. If you wait too long to gather sumac fruit, there may be more insects than flavor residing in the clusters, so do be discerning. Even if your clusters are fairly clean, I recommend straining the beverage to remove the fruits' small hairs.