[header] How to check out a girl without her noticing [title] Shift your gaze around. [step] Staring makes it obvious that you're checking a girl out. Instead of keeping your eyes on her, pretend like you're looking around the room naturally. 
As you're moving your head and eyes around, sneak looks at the girl that you want to check out. [substeps] For instance if you notice her at a sporting event, you can pretend to watch the game but steal looks at her instead.