In conducting the assessment, I would recommend a number of best practices based on my experience performing assessments:
Use a proven template to gather information about the infrastructure.
Employ an experienced architect on the assessment team to avoid common pitfalls as well as provide a comfort level.
Spend time conducting a thorough assessment by conducting interviews and collecting data. Make sure you understand all parts of the enterprise. Using a template covering all aspects of the migration will ensure that sufficient data is collected ”just make sure you do a thorough job.
Remember, this is not the design document, but it is appropriate to make recommendations based on the data collected from the assessment and an understanding of how Windows 2003 will affect the overall infrastructure. This is where experience will pay off.
Produce a document as the final product of the assessment. This will help to summarize the data, detail Windows 2003 advantages that can help the infrastructure, and make recommendations that will aid in developing the AD design document. This document will also help to to review a consultant's work or as a guide for a subsequent migration.