Sometimes, it's obvious by just looking at it. The post seems too good when you check the post history of that particular user. A brilliant post in a history of incomprehensible, hard to understand, and mindless replies. Especially if it's obvious that the user doesn't speak English.

You can just google plagiarism checkers and use whatever you find in the results. Or, take a sentence or two out of a post and run it through a search engine to see if you get any hits. There are also paid and professional apps for finding plagiarized content, but I don't know of any by heart.