One could start with a few well known food items and move from there I suppose.
I.e. milk goes from 4% fat --> 0% fat (skim).
For meat, there are well known measures of how lean the meat is.
For bread, the more grains it has makes it more healthy (I think).
You would only be able to tax the extremes though. For different people, different amount fat, etc... are correct depending on their body types, exercise habits, family history, etc...
It's a good tax too because eating such food increases healthcare costs on the public.
See http://gregmankiw.blogspot.com/2006/10/pigou-club-manifesto....