Standardized symbols for marking food allergies / warnings?
The only true specifications for allergies that I know of are medical ICD-9 diagnosis codes used for medical bracelets and billing at hospitals. However, these codes are not easy to understand, and therefore many people who have food sensitivities or allergies may not actually be aware of them. I would use a listing of common allergens and "-free" after them. (Ex: "gluten free"/GF, "dairy-free"/DF, "peanut-free"/PF, "nut free"/NF, "egg free"/EF, etc). As someone with severe food allergies, I would be able to comprehend this or any obvious images or icons. As long as you make it explicitly clear, you should be okay.

You can find the ICD-9 &amp; 10 codes here:

Specific ICD-10 Codes for Allergies

Specific ICD-9 Codes for Allergies