Summarize:

Ask the butcher if the weisswurst you're buying needs to be fully cooked or just reheated. If you're buying packaged weisswurst, read the cooking instructions to determine if you should fully cook the sausages. Since many packaged weisswurst are precooked, you can simply toss them onto a medium-heat grill. Cover the grill and cook the weisswurst until they're completely hot throughout. Turn the weisswurst occasionally and grill them for 15 to 20 minutes. Serve the hot weisswurst. Avoid poking the weisswurst casing with a fork, knife or tongs to prevent the flavorful liquid inside the sausage from leaking. Prepare a gas or charcoal grill for medium-heat. Brush the grill grate with vegetable oil, so the sausages don't stick to the grill. Place the weisswurst on the grill over indirect heat. Cover the grill and let the weisswurst cook until they reach 140 °F (60 °C). Frequently turn the sausages and spray them with a little water or beer, so they brown evenly and don't split. Remove the cooked weisswurst from the grill and transfer them to a serving plate. Lay a sheet of aluminum foil over the sausages and let them rest for 5 minutes. They'll finish cooking and then you can serve the hot weisswurst. You can store leftover grilled weisswurst in an airtight container in the refrigerator for 3 to 4 days.
Determine if your weisswurst is raw or pre-cooked. Reheat weisswurst on the grill if they're precooked. Grill the raw weisswurst over medium-low heat. Rest and serve the grilled weisswurst.