I think this quite a good, albeit difficult, question to answer.
There is no one "review site" for barristers. So you cant get anything along the lines of TripAdvisor for the bar. You could look up their profile on their website, but hey, that's always going to be good - after all it's one of their marketing tools. You could try and google which cases they have won or lost - but that is no guide either. That wont tell you how many cases they have assisted in negotiating a settlement or had struck out at an early stage. And winning or losing cases is no test since if the barrister goes into court with a good case and then a witness does not come up to proof, there is nothing they can do about it.
My advice is to ask your solicitor a) how much of this work they have done b) how much of this work they have done with that particular barrister c) how they have found that particular barrister.