So my question is, how can I help the community by finding the posts which the user have copy pasted and then show it to the banned person.
Somebody already found it,reported it. that's why the account is banned. Reporters already share that plagiarized post when thread created in Meta.
If there is a method to identify such posts with an ease, then it should be public so that other users could use it to show the banned person for the exact post he was banned for.
Public ? for the ease of plagiarist for hiding the trails?
Why should moderators and senior people take all the burden of finding the plagiarized posts?
Now you want to share the burden but want to copy the method. I suggest you to devise your own method for finding them.