The best way to know when brats are done in the oven is to use a digital meat thermometer, according to the USDA cook the bratwurst to an internal temperature of at least 160 F. Brats won’t be fully browned at 160 F but you can cook them up to 200 F and still have them be juicy inside or you can broil them for 1-2 minutes to get that just grilled look.