Some other things you can do:
1) Check out their books' sales rank on Amazon. It's by far not a foolproof metric, but any book that has an Amazon ranking of more than a million has probably sold less than a dozen copies on Amazon since publication. See how many books have good Amazon rankings (four figures or less) versus bad (over a million). If a publisher has too many books in the bad column, it might be a sign they cannot market their books very well.
You can check this directly through Amazon, or you can use the service at
www.salesrankexpress.com. Not very accurate for strongly-selling books, but scarily so for anything moving only a few copies a year.
Amazon is not the only platform, but it is a huge one. A book not selling well there just might not be selling very well elsewhere. Case in point: any book on Barnes & Noble that does not appear to have a sales rank, probably has not had a sale during that quarter. B & N keeps changing their own rank algorithms, but generally, one sale a quarter is enough to generate a 'rank'.
2) If the publisher claims it is selling books through its own website, you can check how popular it is on the Internet by typing their website address into the search bar at
www.alexa.com. This analytics service can estimate both global and US popularity for a site. Again, the higher the number, the fewer people visited.
3) Stalk the pub's current authors' social media sites and look for key phrases indicating they are hand-selling books, or they have to do all the marketing, or are complaining about low/no sales.
To add to what other posters have said: if you are getting personal rejections and feedback, you may have something strong enough to publish with someone who will actually offer decent editing, book design, and covers, as well as selling copies of your book. You are far closer than many people ever get.
So kudos and good luck!