Look for the weaknesses that most often cause rejection: unsympathetic or flat characters, unrealistic dialogue, slow pacing, a boring beginning, lack of voice, and bad or clichéd writing. You’re probably wondering: How do I know if I have flat characters or a slow pace or any of these weaknesses? Show your manuscript to people you can trust to give their honest opinion, and if they all give you the same criticism, that’s a red flag. You can also consult these two excellent how-to books, which give examples of good and bad writing: The First Five Pages, by Noah Lukeman, and Self-Editing for Fiction Writers, by Dave King and Renni Browne.
One last option is to attend a writing conference or workshop that offers a session or course on revision. Sometimes these sessions are very interactive and feature hands-on editing; other times they’re lecture-based. Either way, they can help you spot and understand your weaknesses in a fraction of the time it would take you working alone.