how can I define what's singular and what's plural in case of ägg?
Singular would have the article ett in front of it.
Think of it like this, you would not say "I eat egg", you would say "I eat an egg", in which case it would be "Jag äter ett ägg"
My friend is allergic, but I eat egg.
I was thinking of the same example. But in this case, interpreting it as "I eat eggs" would mean the same thing.
From context. We have the same issue in English with words like sheep.
For those who got confused by the case/number/gender of "egg", ->http://en.wiktionary.org/wiki/%C3%A4gg
I eat eggs. Lol
So if the noun in plural you do not need an article?
all neuter nouns ending in a consonant are unchanged in the plural. (from wiki). so it's only about the neuter nouns.
You can of course say for example "jag äter två ägg", "jag äter åtta ägg" or "jag äter många ägg" but simply "jag äter ägg" is correct as well.