Scallops require pretty precise timing. Overcook them, and they will be chewy and tough. Although there is no substitute for experience cooking them, here are some tips to help you figure out when scallops are done.
Look for a golden brown on the pan side when you sautée scallops in a pan. Once the pan side is golden brown, flip the scallop.
When the scallop is golden brown on both sides, it is done.
Look for the scallops to break apart slightly along the edge. This works no matter how you are cooking them.
Check the texture. Scallops which are done should come apart easily under a knife.