You've read all the Isaac Asimovs--or all the Jane Austens. Now, you're looking for another sci-fi or another romance. How do you find a good one?
It is easy to judge a genre from the outside, to say, for example, that romances are dumb or sci-fi is all about robots or fantasy is too unrealistic. But the truth is, like with anything, to truly know a genre, one has to experience it.
The problem, of course, is that it takes time and energy (and sometimes money) to get to know a genre. And it means potentially wading through the bad stuff. Or simply uninteresting stuff.
But it is the only fair approach.
I would argue that it takes about 3 to 6 months to really get to know a genre. After that, a kind of "expert" status kicks in.
When I started reading traditional romance paperbacks, my first forays were all over the place. I was reading good stuff, bad stuff, okay stuff, weird stuff, boring stuff. I would go to a nearby bookstore and ignore the snobby comments by the cashier and her friend about how they NEVER read romances (I'm not kidding; you'd think people who ran a bookstore would be more tactful with potential customers).
The initial stage of winnowing involved noting down authors, reading the backs of covers, borrowing books from the library, trying a few chapters. I will tolerate pedestrian writing for a good story but a lack of fundamentals--consistent viewpoint, intelligent vocabulary choices, coherent paragraphs--eliminated a number of writers from consideration.
On the other hand, humor helped as a hook. A fascinating story-line helped as well. Good characterizations were a draw but could backfire if the characterizations didn't match up to the plot.
In the next stage, I began to focus on reading everything by writers I liked. Many of those writers led to others, so one of my first finds, Eloisa James, led to Julia Quin, who had written complimentary reviews of Eloisa James. Eventually, I ended up with Lisa Kleypas. (It was perhaps the other way around, but these authors were all linked by reader and writer reviews.)
After awhile, a kind of sixth sense developed, leading me to authors like Loretta Chase. This sense isn't infallible, of course, and it is difficult to explain. But it honestly isn't all that different from art experts who can spot forgeries without fully explaining how they know That's a forgery. One becomes an expert in a specific type of writing; consequently, one feels drawn to the type of books within a genre that one prefers.
But it all starts by giving the genre a chance to begin with.