We always struggle with hiring a small company for private safaris in Africa because you simply never know what you may be getting yourself into even with a robust review of their website, customer reviews, and more.
Generally speaking, we have almost always found our private tour companies via word of mouth- either from friends or other travel bloggers who have had a positive experience with a particular company.
This is how we stumbled upon Tanzania Journeys for our luxury safari in the country (via our friends Green Global Travel), and after speaking with them about their tour options, we knew we were in the best hands for our tour and booked right away.
To put it bluntly, this may have been the most luxurious safari we've been on (most expensive too), and we'd do it again in a heartbeat given the chance.