I'm convinced that an experienced administrator can guage about half of the traditional teacher observation data points in just 10 seconds. Here's what to look for:
What does it sound like when you enter? Silent? Off-task? A buzz of student learning?
Where is the teacher located when you enter? At his/her desk? With students?
Do the teacher and/or students change their behavior when you enter?
How does the classroom atmosphere feel? Nervous? Confident? Excited?
What are the students doing? Silent? Discussing content? Presenting?
How many students are actively engaged? None? One? One small group? Everyone?
How are the students grouped? Individually in rows? In groups?
Are students looking at you when you enter or are they engaged in their learning?
What is the teacher doing? Nothing? Direct instruction? Facilitating student work?
How is the teacher delivering content? Through an application? Writing on the board? Talking to the class?
What is displayed on the classroom walls? Nothing? Generic Posters? Teacher creations? Student creations?
What lesson content is displayed? Nothing? Lesson? Skill? Standard? Student Practice Options? Agenda?
Bonus Pro Tip: Stand outside the classroom after you exit and are out of sight. What happens in the classroom? An eruption of sorts? Nothing?