You can identify it by try to copy their content 1-2 paragraph or a page to google search or plagiarism checker . If you find many bold sentence on google search or it shows plagiarized on plagiarism checker, you can visit the sites to compare it yourself. But, if you're lazy you can use plagiarism comparison to identify how many % both/multiple text are plagiarized.

I recommended to compare it yourself because I have saw many whitepaper usually do spinning text to avoid plagiarism.