If you spot a suspicious posting (e.g. a fake-seeming testimonial), we'd be incredibly grateful if you'd let us know by sending the URL of the posting to us at firstname.lastname@example.org. Even if you're not sure, we can take a close look and watch out for more problems in future.
But PLEASE don't attack the poster publicly. At best, you'll elicit a huffy denial and start a disruptive flame war. At worst, you may antagonize someone innocent. This sort of issue is best handled privately, via email.
We all share an interest in fostering a discussion with integrity (and one that's non-commercial, focused, and friendly/respectful). So keep those emails coming in; we volunteers count on your help finding problems and keeping things chowy!