How can I tell if competitors are posting fake reviews on my listing?

About 1 year ago sales for our most popular product dropped off drastically. At first I thought it was something with the Amazon algorithm...but now I'm not so sure. I'm wondering now if we were (and still are) being hit by fake negative reviews. My question is, how can you know for certain if they are fake posters? I was looking through our reviews today and several of the 1 star ones were written by posters with no review history at all. When I clicked on their profiles, it didn't even show their review for my product. Does this mean they are fake profiles? If so, how do I even stop this?

Thank you for any help you can offer.