Know what you are looking for. It is essential that you understand what is, and what isn't, a developmentally appropriate activity before trying to pick them out of the preschool setting. DAP activities should include an appropriate blending of teacher- and student-facilitated components that feature play as a primary part of learning. For example, a teacher might introduce basic concepts during a science/art activity on color mixing while the students freely explore, at their own pace, with paints in the primary colors. During the course of the activity, the teacher would ask open-ended questions to encourage the learning processes.
Sit on the child's level and among the class. Instead of being off to a corner or standing outside the room at the door, position yourself either on the floor or a low chair in the middle of the action. This will enable you to see and hear all that is going on. Sitting far away may cause you to lose valuable observation information that is out of earshot or even misinterpret what you are seeing.
Jot down memorable quotes or actions. If you are not the classroom teacher, add notes on how the educator is working well (or not) as a facilitator furthering the activity. Avoid writing long passages that will take time away from the actual observations. Instead use your own, or favorite, type of shorthand and keep the notes to vital points.
Go to a quiet place and review your notes directly following the observations. Elaborate on shorthand notes immediately while you still have the observation fresh in your memory.