If you mean checking how full (or empty) your SWA flight is, some people go by the prices still available. For example if their are only full price refundable tickets left 10 days before the flight, it is basically full. Conversely, if the internet or promotional fares are still available, some folks believe this indicates the flight is fairly empty.
Flying on SWA 15 - 20 times per year, I have found this method to be a very inaccurate way of predicting the load factor.
I sure there will be other opinions posted for you to consider.