How can you know for sure? And if you lawyer up and send a nasty-gram to him and he denies it, how do you plan to prove otherwise? Legally, you'd have to sue and then subpoena the company who posted the review (Google/Yelp/etc.) for IP info, etc., but that's not a cheap option...
So we know it's him for a lot of reasons.
So, for now, the company will remain un-named. Although I'm sure with some sleuthing you'd be able to figure it out. But here we go.
A few months ago, everyone who reviewed us on Yelp received the following message:
"> Steve W. has sent you a message on Yelp:
>
> Did you know this is a One-Man Company?
> "Sterling is the owner, is the technician and is the only person running
> this business out of a house.
>
> Sorry to burst the fake [COMPANY NAME] bubble. You can research all of this
> with a simple Google search. Lucky for you, when you can't reach him, you
> can cancel and go elsewhere."
So I suspected that these letters were being sent out by our competitor, but had no evidence. I left it then, and didn't think anything of it since it really didn't do any damage. I did, however report it to Yelp. The person in question had come to our website and chatted with our representatives a number of times pretending to be someone else, so we have collected the IP address of his business, his home, and various other locations he logs in from. He's also become utterly obsessed with our company. Like, for example, we posted about how secure our site was since the whole thing is wrapped in SSL, and just a little bit later, he posted something to their facebook wall about their incredibly secure website.
Anyhow, suddenly, yesterday, on PissedConsumer.com a review for my company comes up from a guy claiming to have tried to do business with us posted from Boca Raton, FL and another review pops up on Yelp.com from the same location (listed as Delray which is just North Boca basically). This is where the President of the company lives. We've never sent any packages to Delray, we do not have any RMR customers in Delray, and his claim is that he had ordered his service on April 7th doesn't make any sense if you look at our order history. That day, there were 3 orders. Two of them were for product and no service, and one was for service. The customer who ordered service that day is a very happy customer and is still with us.
Needless to say, the review is obviously fake. Both reviews are the first negative reviews our company has ever received, not to mention they came within minutes of one another and were posted under two different names from the same locations articulating two different experiences with our company. One of the reviews, as I said, sounds very very similar to the messages being sent earlier to our customers. So now, not only did he post two fake reviews publicly libeling us, we also have Semantic evidence that he libeled us to individual customers in private messages back in late March.
Since we have his IP addresses, and it wouldn't be all that difficult to discover if he used public spaces or the same computer to access these sites, a simple subpoena to Yelp would be all we need to confirm that its him. A simple subpoena to Comcast or whatever provider he uses would be all we need to confirm his IP and my guess is we could even get his MAC address to discover which computer he used. And I think it's a very simple inference to then assume that the other review was also him even if that site probably doesn't easily comply with subpoenas. So the legal route would be very very easy and quick. That said, I don't think I want to take it. I would rather simply have him remove it or use this as an opportunity to get giant PR on how to deal with unscrupulous companies competing in your niche. Turn a giant negative into a massive positive and throw it in his face.
It means you're doing the right thing and leaving the competition in the dust. I would personally send him an email and ask what's up, why he's going after such methods and why he won't do a fair battle in the open market. 
