In addition to seeing hidden sources of shellfish and fish here, here are some tips:Hidden sources for Fish/Shellfish: do you know of any?
I, almost without exception, do not eat grilled food or fried food at restaurants. The are exceptions: a dedicated grilled or fryer for shellfish. And that's typically if both the manager and the chef confirm it.
When the staff says, "We clean it daily," I ask, "With what?" They use water, folks. Water. That won't get rid of the shellfish protein.
When I order, I typically use wording that we rarely do when ordering things for peanut allergic DS. I always mention "cross contact" (less offensive than "cross contamination") with utensils. That's because my worst reaction was from tongs used to turn shrimp. I honestly don't worry about that as much with peanut.
Actually eating at a restaurant typically involves ordering something as low risk as possible. Not ordering what I would like to taste or even ordering what is healthy. Especially not that, as sometimes the safest food is something that has cooked in its own dish--baked. And often that means something with a lot more sauce than I'd ever use myself.
I don't buy fish in the grocery stores because I see cross contamination there all the time--in the windows. I have no confidence in their handling fish safely, and I don't know the supply chain. Now that I live in a city again, I assume I can buy fish at a Kosher market. That should be safe.
What do you do to avoid shellfish cross contamination?