Check the internal temperature with a meat thermometer
Pork, beef, and lamb sausage: 160°F / 71°C
Poultry sausage: 165°F / 74°C
Cut into the thickest part and check that the center is no longer pink
Juices should run clear, not red or pink
The sausage should feel firm, not soft or squishy
The casing should be browned and evenly cooked
Cook until the sausage is hot all the way through
If unsure, cook a little longer and recheck the temperature