How can one be sure those subreddits don't have any paid posters?

When you have a popular post that reaches the Reddit front page, you may receive messages from people asking if you'd like to promote their products.

My guess is that they eventually will (and I'm sure some already do). However, one thing that kind of helps weed out the fake reviews is the fact that you can inspect a user's post history.

On reddit or other more general sites, you can see their entire comment history. Hopefully you'll notice that a tiny tiny portion of their comments are related to products or reviews.

In other words, you can get a feel for if the person is genuine or not.

Of course, that doesn't 100% rule out paid shilling by (otherwise) honest people, but you do get a much better sense of the veracity of reviews on a site like reddit than you could ever get from Amazon.