Author's co-writer here – there are a few common ways to get this information from public sources. Public documentation, press releases, analyst reports from places like Gartner or G2crowd, field reports from a sales team in an enterprise business are some examples.
They occupy a really strange place in the industry – relied upon by many, but heavily conflicted in terms of their incentives. The ritual and strategy of participating in something like a Gartner magic quadrant is worth a whole post in and of itself.
Thanks for reading!