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In most recently published articles, the DOI will be printed with the article itself, usually on the first page somewhere, or in the header or footer.
For all articles, if the DOI isn’t on the article, you can look it up on the website crossref.org.
For Chinese articles, you can research on chinadoi.cn or www.wanfangdata.com.cn. These free websites allow you to paste all or part of a reference to find the corresponding DOI.