The genuineness and originality of the articles published in a journal must be verified by the publisher. Genuineness and validity may be revealed during the peer review process. Plagiarism can be detected by the use of good plagiarism checking softwares. The publisher/reviewer must ensure that all portions taken from previous works are properly acknowledged.
as per my knowledge it depends on different conferences and journals. each and every conferences and journals have their own set of standards and rules. depending on them what standard they want to maintain. there is even a concept called self plagiarism where we refer to our own paper as reference. generally these guidelines are not disclosed by the conference and journal people. to revert back they will only state that the rejection of paper is done due to plagiarism and lack of originality.
Just making sure the work is that of the author and not some one else's. For this purpose, the reviewer is required to have extensive knowledge of the material he/she is working on. Only citing through proper means or quoting is a right way to utilize an idea of some, not as own. There has been a through discussion on this issue here in the link I have posted and a study made in the next link where in section 3.3.3 of the finding outlines what plagiarism might be in terms of numerical values.
The genuineness and originality of the articles published in a journal must be verified by the publisher. Genuineness and validity may be revealed during the peer review process. Plagiarism can be detected by the use of good plagiarism checking softwares. The publisher/reviewer must ensure that all portions taken from previous works are properly acknowledged.
The term plagiarism comes from the Latin word for kidnapper, plagiarius. It means taking credit for ideas or words that aren’t your own. It’s a form of stealing. A person who plagiarizes another’s work is committing theft of intellectual property.
COPE is a forum for editors and publishers of peer reviewed journals to discuss all aspects of publication ethics. It also advises editors on how to handle cases of research and publication misconduct.
Avoiding plagiarism, self-plagiarism, and other questionable writing practices: A guide to ethical writing --- see link