Anna Kristel, writing at her author blog, offered this piece of advice on how readers can detect piracy:
What prompted this post? A few weeks ago, one of the authors who writes for one of the pubs I write for happened to find almost all of our authors’ books on a couple of sites. We were all just sickened by the news and immediately sent letters to the owner of the sites. We also asked friends to do the same.
I decided to address this issue to bring attention to it to unsuspecting readers who may not even realize they are downloading books illegally. The rule of thumb…if it’s not one of the major retailers…it’s probably not legit.
It's difficult to convey just how bad that rule is, but I will try.
Following this rule of thumb could lead you to conclude the search engine site Luzme or the discount ebook site eReaderIQ are pirate sites. After all, they list vast quantities of free and discounted ebooks and neither site is one of the major retailers. The fact that neither site hosts any content but instead links to major retailers would be easy to miss if you don't look too closely (I've made similar mistakes before).
Anna's rule of thumb is also how the Lendink lynch mob happened. Back in 2012 a few authors discovered that a nondescript little website was listing their ebooks as free. Panic ensued, and before the day was out thousands of authors flooded the site with DMCA notices.
Unfortunately, none of those authors looked closely enough to realize that Lendink didn't host any content. It was one of many legitimate sites that were set up to facilitate the ebook lending feature offered by Barnes & Noble and Amazon.
In short, it's simply not possible to apply a rule of thumb when you are trying to determine whether you're looking at a pirate site. The internet is far too complicated for a one step rule like the one Anna suggested above. Trying to follow that rule of thumb is probably going to result in a bad outcome, and it might even lead to another fiasco like the Lendink lynch mob.
I would suggest that it it better to look carefully, ask questions, and make sure that you understand all of the technical details before reaching a conclusion. Given how much damage a false accusation can cause, I would say that it is better to be safe than sorry.