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.

This is always the way: once something has a large enough audience it will become a target for exploitation. This is why ad networks are juicy targets for hacks (wide spread of potential victims) and popular sites, or specific areas on those sites, like reddit can quickly become overrun by shills.

If the thing is open and understood, then I will have a much better time. If not, it will probably soon go on the abandoned trash heap.

This works especially well if the domain is not niche, but has wide general-population applicability. Say, dieting. The amount of content-marketing bullshit on the general Internet makes general topical searches near useless. Whereas just browsing the collected knowledge (pinned posts, wikis) of various dieting subreddits let me quickly find one appropriate regime for me, and then its specific subreddit taught me how to apply it safely.

(Yes, it was /r/keto, and yes, it worked.)