Which food preservatives or additives on salads in restaurants cause diarrhea about an hour after eating and is there a way to tell if these additives are present before eating?

Diet dressing - sorbitol, xylitol or mannitol

Sugar alcohols are fairly sweet but aren't digested well and some people can't digest them at all so have extreme symptoms. Sorbitol is also prescribed in pharmacies as a laxative but the dose is similar to eating 3 to 6 diet ice cream bars at once.

Low fat - xantham gum, guar gum or inulin.

These are high fiber, thickening agents. They give a high fat-like consistency and prevent oil and vinegar from separating. People have differing gut bacteria ecosystems and, for some people, these additives will cause quick diarrhea symptoms. They are considered safe but some restaurants, that make their own dressing, may use far too much above 0.15% Xantham.