It’s a pity the search engine doesn’t seem to be working right now. I asked this very question a year or so back, led to a informative thread.
Any scallop you buy at the store will be a real scallop, unless specifically marked as imitation scallop. Any reputable restaurant will serve scallop, not skate. Serving skate flesh as scallop is something that may have been done by shady sorts in the past, but it’s going to be rare these litigous days.
It’s pretty easy to tell by looking at the grain of the meat if it’s real scallop or not. Real scallop muscle is composed of many short strands running paralell to the axis of the cylinder of meat, with no apparant side-to-side grain or flaking. Fish flesh will have a side-to-side grain and may be noticeably flaky.
I hear that over in England you can by scallops with the (apparantly quite tasty) roe still attached. Obviously, that makes it easy to tell it’s not fake.