Whatever method you used to cook your ribs low-and-slow, you can move them to the grill at this point. Heat an oiled grill to about 225F (107C) or a bit higher before setting the ribs over the direct heat. You can keep using the same marinade you've been basting with all along. However, you can also finish with a barbecue sauce. Brush the sauce on every 5-10 minutes to start building up a thick coating on the outside. You can use store-bought barbecue sauce or make your own. Flip the ribs occasionally during this process, every 10 minutes or so. That way, the ribs will heat evenly on both sides, browning the sauce as you go. To check for doneness, pierce the meat with a fork. It should slide through the meat fairly easily. Let the ribs rest for 5-10 minutes under tented foil. Use a sharp knife to cut the rib rack into individual ribs by cutting the meat between the bones.

Summary:
Move the ribs to direct heat on the grill. Baste with barbecue sauce or marinade. Cook the ribs 20-40 minutes more to reach peak tenderness. Slice the ribs between the bones.