!howfair is calculated by the K/D's (Kills per deaths, in silentmod stated as KR) the different teams have.
in Silentmod you can tap the "TAB" button twice to toggle between 3 different player stats when holding TAB. so for example: you double press TAB quickly, you can see the PR (playerrating, something about the map victories-losses of players). if you double press TAB again you can see the KR rating of the players, which !howfair uses to check if the teams are fair.
The best an admin can do is not only check !howfair, but also do /scores in the console, to see if the kills and deaths of both axis and allies are in balance. (note that they don't have to be in balance, since maps have an impact too. also note that big difference in these stats on servers where 50 players are online, might not be a big difference at all).
Edited: Link removed