You don't, though if they were, it's very unlikely they would be doing it in bulk for every file, since someone would notice at some point, either by inspecting the file or notice that a file when downloaded to a canary system misbehaves.

Targeted attacks might be possible based on the file contents or using a side-channel to ID the downloader; meaning only specific targeted users would get the malicious files.

Either way, being able to hold Sci-Hub accountable as this source would be important.