There are tools like turnitin and grammarly but these are academic and don't check against bitcointalk. Google results have dates on them and so do BTT posts, i guess that is the best way of determining who the original was
Checking plagarism test is better to use Google description of checking plagarism, in universities we use Google to check every work frame of students and especially during project defense and seminar presentation of any students, if you process these very well is okay that no difference between getting bitcoin plagarism and academic plagiarism, so with Google you get it clearly.