This is a routine part of the editing process. I wouldn't tackle line edits without first making sure I understand something about the voice.
So first, I think about the overall tone of the piece and how it might shift over the course of the story as the scene content changes. Is it generally light and breezy, with brief shifts into something more dramatic? Or is it dark and contemplative throughout? Is the prose direct and clean during action scenes and more stylized during erotic scenes, dream sequences, and the like?
With that in mind, I take a careful look at the actual sentences on the page. I'm looking at:
- How an author uses verbs and action words. Does she pack them in or use them sparingly?
- How she handles simultaneous actions.
- What types of introductory phrases she might throw in to alternate sentence rhythms over the course of a paragraph.
- Does she use compound sentences? Complex sentences? If so, when and where?
- How she uses prepositional phrases. Does she chain them or use them sparingly?
- Adverbs and adjectives. Some love them. Some hate them.
- Vocabulary. Some writers use very simple language that could be readily grasped by a child. Others make me continually reach for my dictionary to check nuances.
- How frequently does she name her characters instead of using pronouns?
- Length of paragraphs, use of offset paragraphing techniques, balance of dialogue to action in paragraphs.
- Sentence subjects. Is the subject always a character? How frequently is the subject something other than a character?
All of these things, and probably more I can't think of at the moment, contribute to the mechanical aspects of voice. And I have to understand these things in order to complete a line edit in what we hope will be a competent fashion.
Let me give you an example. Say an author starts her sentence with a present participial phrase containing an action which should be sequential and which cannot modify the subject. Alicia, I'm going to borrow your classic example here:
Tying her shoes, Mary ran up the stairs.
My goodness, but Mary is talented. I can barely walk and think at the same time, and here she is running and tying her shoes at the same time.
What the writer means to say is that first Mary tied her shoes, and then she ran up the stairs. These actions are sequential, but they are presented as simultaneous. Not only that, but participial phrases are modifiers, and this reads as if it's not intended to be a modifier but a verb.
To edit this, I would want to understand how the author generally handles sequential actions. Does she use compound verbs? Or does she prefer compound sentences? I would try to stay closer to her first preference. If doing so creates other problems in the prose -- such as repetitive sentence structures -- then I might reach for her second choice. Whatever I do, when fixing the error, I would be doing it in a way that preserves voice.