America's test kitchen covered this in an episode not too long ago. Let me go by memory here...
As other have said: Sodium Trypolyphosphate is added to most scallops to help them retain moisture and it does make for an "off" taste. The bad news: There's nothing you can really do to pull the STP back out. The way they said to test it is to dry the scallop thoroughly, put it on a paper towel, and microwave it ever so briefly. If the paper towel is wet, you have a treated scallop.
ATK's solution was to marinade the scallops in water and lemon juice. It doesn't pull out the STP, but goes a long way to mask the flavor.
[ETA] I actually found their writeup and could have save my typing above:
We strongly recommend purchasing "dry” scallops (those without chemical
additives). If you can only find "wet” scallops, soak them in a solution
of 1 quart cold water, 1/4 cup lemon juice, and 2 tablespoons table
salt for 30 minutes before proceeding with step 1. In step 2, season the
scallops with pepper only. If you are unsure whether your scallops are
wet or dry, conduct this quick test: Place 1 scallop on a paper
towel-lined, microwave-safe plate and microwave on high power for 15
seconds. If the scallop is "dry,” it will exude very little water. If it
is "wet,” there will be a sizable ring of moisture on the paper towel.
(The microwaved scallop can be cooked as is.)