Well a good way is to look for a site which specializes in independent reviews.
One such which specializes in accommodation and dining, is "tripadvisor".
I wasn't sure if they also covered reviews of tours but just in case I Googled for
tripadvisor thedragontour and indeed they do cover it:
The Dragon Trip ... Ranked #15 of 39 tours in Shanghai
Now the next question is whether tripadvisor's reviews are reliable. You should ask this because in fact this particular tour gets 12 "excellent" ratings from 12 reviews. No reviewer has given it any rating other than the highest possible.
There are ways to check whether reviews are genuine and not paid for or maid by friends of the company's owner:
- Do the reviews go into particular details or just say "it was great"? Look for things that make it sound like the review really did go on the tour.
- Do the reviewers have other reviews on the site? If so check some of those. Does it look like they've been travelling around? Have they also given bad reviews to competing tours? Do they always give "excellent" ratings to everything?
I only had a quick look around but several reviewers look like they've reviewed other things in other places and not always favourably. You should take a deeper look for yourself but I have the impression that people really do think this is a great tour.