If it is a continuing flight it is easy as others have offered above.
If it is the beginning of a flight (new flight number) you are trying to figure out where the plane assigned to that flight is coming from to tell if there might be a delay it is more difficult.
First you look at your flight on the day it is flying before the week you are going and see what gate your flight typically leaves from. Then look at other SW flights into the airport and see what flight arrives at the same gate as the last arrival before yours leaves and look were it is coming in from. Not full proof, but I have found it is pretty accurate.
I use this mostly during the winter to try to pick a flight that comes in from a warmer climate.