Seldom are allergen ingredients obvious to the point of being readily identifiable with a quick glance at a plate of food. Sure, you could print up little menu cards that list major allergens (or hope that the host does this...). But you could also use visual cues to clue in a diner that a dish contains a caution ingredient.
Ming Tsai - acclaimed chef, owner of Boston's Blue Ginger restaurant, host of Simply Ming, and outspoken food allergen advocate - gave a useful example a few weeks ago on his TV show. He was making a pad thai dish, which included peanuts and peanut butter in the sauce. Once the dish was prepared and plated, it wasn't immediately apparent that it contained peanuts. So he garnished with coarsely chopped/crushed peanuts. It was a visual reality check / reminder for anyone with a peanut allergy. (In this case, the peanuts also provided an additional texture to the dish.)
Shirley, our GF blogging buddy at Gluten Free Easily, recently posted another example. She was sharing her recipe for a crab ball, which she likes to serve at holiday parties. In the past, she has had guests who have had shellfish allergies. And so Shirley serves up her crab ball in a serving plate that's shaped like a crab. In this case, the visual reality comes in the form of the plate itself, rather than a garnish applied atop or next to the food on that plate. But the effect is the same.
Of course, as gluten-free foodies, the defining element of our dishes is that they lack wheat (and barley and rye). This complicates the whole visual reality thing. You can't exactly garnish a dish with a stalk of anti-wheat to remind diners that your food is gluten-free. But then again, that's not so much the concern here. What we're really worried about is making diners aware of what IS in a dish in the event that they may have a life-threatening allergy to it.
It's not always practical or realistic to use such visual cues. Especially when a dish may have multiple major allergens, or the dish doesn't lend itself to certain in-your-face garnishes as reminders. But it's a useful technique to keep in mind when you're hosting large groups of people. To the degree that you can reasonably do so, use visual cues to tell guests what's in your dishes. Whether it's nuts or shellfish or any one of the other 8 major allergens defined by the FDA, your guests will thank you for that extra little bit of effort that lets them eat safely and with confidence.