If you think a video on Douyin has illegally reposted someone else's content, you can take the following steps to check and report it:

1. Open the Douyin app and log in to your account.

2. Enter keywords on the homepage or in the search bar and try to accurately describe the content of the video you think has been reposted.

3. Browse the search results and find content related to the video you think has been reposted.

4. Click to enter the video page, watch the video and verify whether there is any illegal reposting.

5. If you confirm that there is any illegal reposting, you can click the "Share" button in the lower right corner of the video and select the "Report" option.

6. On the report page, select the appropriate report type (such as infringement, vulgarity, illegality, etc.), and provide detailed reasons for the report and relevant evidence.

7. After submitting the report, Douyin's review team will review the video and take appropriate measures based on the situation.

Please note that sufficient evidence and reasonable reasons are required to report videos so that Douyin can handle them effectively. At the same time, Douyin also encourages users to actively participate in maintaining the order of the platform and jointly create a good network environment.