Assessing is not just something one does at the end of a unit to determine whether or not learning goals were met. Instead, assessment is something that should be done throughout a unit, and even on a daily basis. It should include not only traditional quizzes and exams, but projects and performance tasks as well. In this way, it is easier to determine if the student met the learning goals.
What I do to help me assess student learning is to keep a clipboard close by at all times. This way I can take notes on my students during certain activities. This sort of data collecting is helpful in that it allows me to determine on a daily basis where my students are and where they need to go. It also allows me to group students based on their shared needs. Documenting their answers and my analysis of their work allows me to go back and group my students to work on target goals.
Based on my data collection, I break up my students based on shared difficulties and learning goals. I work with those groups of students during my Daily 3 time, as well as during math centers. This has been very helpful in my really understanding the needs of my students, and meeting those needs as well.