[j.p.s]

I did a bit of looking around and found a couple more ways to do it.

1. qpdf gives a bit cleaner results.

for f in *.pdf; do qpdf --show-encryption $f > /dev/null; done

2. For those like me that find perl easier to read and write than python

#!/usr/bin/perl
use PDF::API2;

while (glob "*.pdf") {
  $pdf = PDF::API2->open($_);
  print "$_ is encrypted.\n" if $pdf->isEncrypted();
}

PDF::API2 was not included by default on any of my systems, but neither was PyPDF2 including on a very large anaconda install of python at work.