This is a good question, and it is definitely a hard thing to assess.
That said, I usually look at their customers and judge by whether or not anyone notable finds their services worth buying, and how many notable people are buying in.
Distimo is owned by App Annie (an acquisition IIRC), and their clients include most of the biggest mobile companies from almost every region:
https://www.appannie.com/about/customers/?_ref=header
Part of the reason for this is that if they're customers for the service, and they know their own data is wrong, they can have a large hand in helping correct that until the estimation models are quite good, since it benefits them by ensuring they get good data about how others are performing.
This is why I tend to default to their research more than others.
NewZoo also has a very strong client list:
http://www.newzoo.com/clients/
Superdata is quoted a lot on the forum, and they do have a targeted base with quite a few relevant players, but it is certainly less expansive than the others in that it's a lot heavier on just traditional vendors:
http://www.superdataresearch.com/about/
When looking at their data, I'm generally a bit more skeptical unless it's covering things their own clients are notably involved in.