Scallops need to be big enough that they don’t fall through the cooking grate, so look for bay scallops or sea scallops that are at least 1 inch wide (never the tiny, rubbery calico scallops). Scallops should be cream colored or slightly pink. The bright white ones have been soaked in tripoly-phosphate and water. As soon as they get hot, the water runs out and prevents browning.
Often the large abductor muscle, which was once attached to the scallop’s shell, is left on. Peel it off and discard it. It’s chewy.