What references must be acknowledged in a paper? This question came up from a discussion on meta.MSE.

My question is: 


  Do we need to search MSE (or blogs, math forums, ...) to make sure someone hasn't already proven a result when writing a paper? 
  
  What if we are already aware of a them (so no need for searching)?
  
  Is not citing such a post in these two cases considered plagiarism?


As I understand, the common practice is to check standard reviewed reputable publication venues (journals, conferences, maybe arXiv) and also with experts in the area to make sure a result is not already published nor a well-known folklore result. No one is going to search all over the internet and check every post that Google returns and citing other resources is very uncommon. I think checking MatheOverflow can be considered similar to the later (checking with experts) (also see this discussion on MO but that doesn't seem to apply to a site like MSE. I am not going to cite a discussion with some random person on the street (not a professional mathematician) who claimed to have a solution or an idea for a solution for a problem (which is not passed peer-review process and I might not want even want to spend time understanding or checking the correctness of the solution).


  What are the accepted practice for checking originality of a result? 
  
  What is expected from authors regarding this before making a paper submission?




Some clarification since there seems to be a misinterpretation of the question about being academic honesty. The question is not about posts that


you are aware of,
contain a complete rigorousness solution (not just ideas), and
you are confident the solution is correct.
This is a question of academic honesty and due diligence.

If you did not arrive at the result yourself, but got it from someone else (either on SE or a homeless man on the street) then it is your responsibility to not claim credit for the result. Of course, in the case of the homeless man you can get away with claiming credit, and in most cases for SE, too. However, it is simply not honest.

If you arrived at the result by yourself, then as a research you should provide due diligence and check if the result is already known. This usually consists of checking the standard sources (i.e. published papers, books) and the communities which you are a part of. "Folklore" in mathematics is vast, and MO, math.SE, and cstheory are all becoming part of it. If you are aware that others arrived at a result before you then you should mention it in your paper (either with a full citation, if applicable, or with an acknowledgement). 

However, just like you are not expected to search the back-log of every journal ever published, you don't have to scour the whole internet, either. If you want precedent of this: consider all the results that were published independently in the west and the soviet union during the Cold War. It would have been unreasonable of the scholars on both sides to be fully aware of the work of the others.