How to avoid generalization?
Generalization should be encouraged to extend scope, thereby making academic papers more widely applicable. Perhaps the OP is actually interested in another question: It is unclear what the OP means by generalizing, I don't see a relation with understand[ing] the difference between fact and opinion and try[ing] to express things with different levels of certainty. Maybe the OP is interested in 


  find[ing a] way to differentiate between fact and opinion...It would be great if someone can explain the difference between these two things.


Let's look at the definitions (from Google) of those terms:

 Fact. A thing that is known or proved to be true, e.g., hedgehogs have fleas.

 Opinion. A view or judgement formed about something, not necessarily based on
 fact or knowledge, e.g., hedgehogs should drink milk.


So, facts are true, whereas the validity of opinions are unknown.



The first example is also from Google, the second is a widely believed opinion, which is false.