In simpler times, it was easy to tell if a bathroom stall was occupied. One needed only to crouch down and look for a pair of feet under the door. These days, however, it's not always that straightforward. Fancy new bathroom designs where the stall door goes all the way to the bottom leave no room for the crouch and spot. This can create some awkward run-ins with colleagues, roommates and complete strangers for those who choose to simply try the door.
To get around this, knock lightly and listen for a response. Trying the handle should be the last resort. Some stall doors don't lock very well, which makes barging in a roll of the dice, and lead to embarrassing situations. You wouldn't want to run into your boss, would you?