Standard pork sausages take around 30 minutes in the oven when cooked at 200C/400F. This recipe bakes them for 40 minutes, so you can be sure they will be cooked through.
To tell that your sausages are done you can either use a meat thermometer or cut into one and check it's cooked in the middle. They should be steaming hot and firm in texture in the middle, not mushy. If you use a food thermometer then you want the sausage to be 75C/165F when you remove them from the oven.