Since the filename is an internal server ID, there isn't a public "directory" to look it up. However, you can try these steps to find the original post:
When you download a video directly from Reddit's servers or use certain third-party downloaders, it retains this alphanumeric "source" string as the filename. How to identify the content 0h8axkjfx83onf4nz4skv_source.mp4
If you can take a screenshot of a clear frame from the video, use Google Lens or TinEye to find the original thread. Since the filename is an internal server ID,
The filename is a unique identifier typically generated by Reddit's video hosting service (v.redd.it) . 0h8axkjfx83onf4nz4skv_source.mp4
If you recently downloaded it, check your Reddit "History" or "Saved" tabs in your profile.