How can I plan on-going student-centered assessment?

Assessment is not just testing what the student learned at the end of the project. Assessment needs to be a tool that I use to make sure that the students are traveling in the right direction and acquiring the knowledge from the project.
Assessment comes in all forms. Monitoring their progress, asking them questions on the subject, and finally grading their end result.
I must stay involved throughout the project to ensure the students are not only learning the subject matter, but also practicing the 21st century skills.