As I have gotten older and (I hope) a little wiser, I have come to realize my first attempts were more content-focused than skill focused, and that I really didn't know how to plan my instruction based on the data I gathered from those pre-assessments due to raw teaching inexperience. Plus, when my pre-assessments revolve around how much students know about a topic at the high school level, I usually already know the answer--not much. Why waste instructional time giving students a pre-assessment over a topic when you know that they will do poorly, causing frustration all around?
What I have discovered works best for me to plan for differentiated learning is to pre-assess students based on their current thinking skills rather than on how much content they can recall. In other words, I now like to give students a task where I can see what type of thinker they are, or to what extent they can think about the topic given to them (and usually the extent is measured along Bloom's taxonomy). This way I can better differentiate their learning based on the complexity of the task at hand, while evidence for mastery of the standard still stays the same. While I haven't perfected this pre-assessment process yet (and I'm not sure that's entirely possible), seeing how well students can think about a topic before any instruction has really helped me better plan for the diverse needs of my learners.
To see an example of this, you can check out my pre-assessment for my Science Inquiry Unit. In this pre-assessment, students will design an experiment after observing a demonstration from me. I will be "closed" for questions regarding how to design their experiment or to answer questions about content, and I will only be "open" for minor questions. While teams are doing this activity, I will be walking around the room, taking notes on each student using Evernote using my iPhone, and planning for my next steps for the unit right then and there. It's basically a gigantic formative assessment day, where the data comes in the form of useful observations of student thinking skills laid out before my very eyeballs.
I also give students this written pre-assessment in a Google form. It is differentiated in that, based on student responses to the first question, it takes them to a different middle question that allows me to see what they know at their current level. All students are then redirected back to the same last question. I do this along with the experimental design pre-assessment because, as we all know, how students perform a task in front of you and how they interpret and reason when answering written questions can be at two very different skill levels.
Like I said, I haven't perfected this type of pre-assessment yet, so any comments, suggestions, or other methods of pre-assessment you have used successfully would be greatly appreciated.