You can take a guess about the size files should have.
Also you can rule out most zip files.
If your searches don't turn up regular results, you'll often get only spam, and the second best way to work with that is not to share them. We can't yet block all spam, sadly.
The best way would be to join development (or find someone with Java skills who'd like to join) and implement credence or a similar verification into phex:
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Credence: Thwarting P2P Pollution