The most safe and sure way to know when smoking sausage is done is to go by its internal temperature. Not only is sausage unsafe to eat if it hasn't reached the recommended temperature range of 155-160 degrees Fahrenheit, if it goes over that it begins to lose juices, becoming dry and tough. The fat in the sausage binds the meat together and has lots of flavor. When it melts away, what your left with is crumbley and lacks taste.
The United Stated Department of Agriculture (USDA) recommends 160 degrees, while the National Pork Producers council recommends the lower temperature of 155.