I don't know if there are any really good ways to do this that aren't a total crap shoot. I believed people on SB who recommended dive operations as being fantastic and got seriously burned. I looked at dive operation web sites and found them outright lying with claims and their videos had nothing in common with the actual diving I experienced when I went. I have also researched on-line and picked on my own and got less than wonderful results.
On the other hand I have picked on my own after researching Bonaire and everything was perfect. I don't think I could have made a mistake going there, staying with anyone, and using any dive op. But that's just what I like. I also picked on my own after being on an island and having some idea what was there, what the island was like, and what the diving was like to give me some background to evaluate what was found on-line and from other people and had multiple wonderful experiences.
The difference between a great experience and disappointment had almost nothing to do with what other people like (what I want out of diving might be far different), what the reputation of the dive op is (the rep is built by people who love the place not objective observation), what the reputation of the location/country/island is (tourist promotions for everywhere claim the best everything). And absolutely do not believe the dive operation's web site or claims. What they put there is the absolute best days, in the absolute best conditions, from the absolute best year - and that could have been 20 or more years ago. They might also just outright lie - happened to me once, even though I extensively emailed back and forth.
What does make a difference is knowing exactly what I want, and then asking the right (specific) questions of people who have been there - preferably several times. I need to have a clear idea of what I want in a dive, of what conditions I require, and of what I want/could expect to see, etc. Once I know that, I can find out if my expectations will be met by doing a lot of digging. Too many times I got caught up in general claims of "fantastic". "best place in the world to dive with X", "best diving in the world", "world class diving". Eh - not so much.