Watching different sports for many years (and listening commentators), reading the articles and writing about it. You can google it. Also, there is a statistic about countries medals at big competitions, and you can compare country results at home with the competitions at neutral ground. Now with more technology involved to help judges to make the right decision, it's less likely for them to judge in home country 'favour' in those 'delicate' situations (where you need to give yes or no decision and there were 50% chances for both things to have happened in 'your eyes'. I'm not saying that judges premeditatetly are judging in some favour, just in complicated situations where they are not sure about objectivity of it).