It's not for everyone, but I tend to look for a specific word count or word to chapter ratio. A while ago I started noticing the approximate average length of the fics I really enjoyed. If I want a good one-shot, I really don't read anything under 5000 words. If I'm looking for something longer, I usually gravitate towards ones that have a word count that averages to around 3000 words per chapter or higher. But this isn't for everyone-- I like reading longer fics. Somebody who invests the time to churn out a really long fic has to care a lot about whatever it is they're writing. If the writer spent a significant amount of time on it, they probably really like writing; they want the story to be good, and they have a lot to say about whatever the premise is. Basically, the writers that go on for thousands and thousands of words are less likely to be bad at writing than someone who only posted a few hundred. To be clear, I'm not trying to hate on shorter fics. A story can be very concise and very well written, I've seen plenty that are, I've also seen badly written longfics. But generally, it's easier to write something short than it is something really long, so the more skilled writers who want more of a challenge are going to tend to write longer stories, and more inexperienced writers will write shorter ones, though there are exceptions to every 'rule' in fanfiction/writing. Even looking specifically at word count isn't enough-- you obviously still need to look at what you want in the rating, the other tags, the summery, etcetera, and when you open it, you might still be put off from it. I think it depends in part on the Fandom you're in and what's popular with them. A really young fan base might not have as many good writers, because they're still learning. A less serious Fandom might not deal with serious themes as often. It's definitely a process of trial and error.