When Southwest was using an older computer system several years ago, it used to be very easy to glance at the gate agent's screen and the number of open seats was pretty prominently displayed! (Or it might have been the number of checked in passengers, and you could calculate it pretty easily!) I don't think it's so easily viewable now. If I'm curious, I'll now tend to ask the flight attendant by the boarding door who usually gives a pretty good estimate of how full we are.