When you "dip" into their reading, sit down next to them and ask them to read aloud to you right where they are. It doesn't take long to figure out if the book is a good choice for them...are they reading at a pretty fluent rate? Or are they stumbling over their words? Do they seem interested? Or obligated to it? Give a quick "tip" to the reader and then move on. A tip could be a question you ask to nudge them further in their thinking. It could be a suggestion to abandon the book. Or it could be a strategy for figuring out an unknown word or part of text.
Although you could probably give a lot of tips, resist the urge to do so. Give a quick one and move on. This allows you to get to the next reader. It also allows your readers to continue their work without a lot of interruption.
You could carry a small notebook or note-taking page with you to jot down the tips you are offering each reader. This provides a good anecdotal record over time of the student's independent reading behavior.
How do you conference with your students during independent reading?