how to find whether this paper is already available or not, if authors just changed the Paper title & author name? I am the editor. I received a paper from some author. I need to check whether that same paper is already published in some other journal or not. For example some authors downloaded a paper from some other journal, they just change the paper title, and author name and they send that paper to us. So in this case how can I find whether this paper is already available or not?
There are several software available for checking plagiarism, which would also fit the problem you are investigating. The journal (actually the publisher) for which I am editor use iThenticate. I am not in a position to compare the numerous software and services around, some free, some commercial. However, a simple check that can easily be made is to to a Google search on bits of text. I have personally used this to check on similarities between manuscripts and earlier published articles to check on the degree of overlap and it works fine. It will give you a first indication at which stage you can decide on continuing with a more advanced analysis. 

See also answers in What is the easiest way to check for plagiarism in student essay papers?