First, your own writer’s instinct. If you doze off at night and wake up three minutes later with an unscratched mental itch–do the conversations between Freddy and Raú l ring false because their voices sound too much alike?–chances are you need to at least jot a note on the pad beside your bed. (You do have one there, right? And one in the bathroom drawer and another next to the coffee maker?) You may even want to (or be compelled to) get up and revisit the boys in the magical realm of your computer screen.
Second, other people’s writers’ instincts. Read the comments that your critique readers have given you. Consider each one carefully. Then consider them as a whole. Is there a pattern?
True, this can create some initial confusion. Some people will like the character of John–the fifty-one-year-old tech services geek who moonlights in a biker bar–while others will think the story could better be told from the point of view of Angela–the twenty-seven-year-old police dispatcher with a passion for antique glass ashtrays.
Stephen King says that if you get a split decision, you can call it a push and make your own choice. After all, you’re the writer, and you know (or should develop a sense of) your goals: the territory you want your want your piece to cover, the emotions you want to elicit, and the best way to do it.
On the other hand, if most people weigh in on one side of a question–say, too much backstory too early in the piece (a flaw of mine)–maybe you ought to rework it with an eye toward focusing on the action and weaving the information in later–or cutting it. Try taking people’s advice as an experiment and see what happens.
By the way, if you don’t have people you trust to give you honest, useful comments about your writing, maybe it would be a good idea to check out the local college and adult-education writing classes. Librarians often know where critique groups meet (libraries are a favorite venue), and sometimes people who attend writing conferences are looking for new blood in their groups.
Meanwhile, keep in mind the underlying idea that revisions should have at least one purpose. This can keep you from defaulting to line editing–again–and help you to spend your writing time effectively.