I normally say during a talk that say to the server even a wheat crouton lifted off my salad will give me the worse case of food poisoning/throwing up immediately (bit of an exaggeration for some, not all) and they understand.
I know we're all different in how we react. One of my concerns is that my reactions generally come an hour or two after being glutened, and I fear that if someone messes up (intentionally or not) and doesn't see me running into the bathroom immediately following, that I was 'faking it'. This only reinforces their carelessness (intentional or not) for the next person with Celiac or NCGI. I tend to say, "I won't react here, but it will be very, very bad at home and for the next three weeks".