A good indication is found by looking at the Scandal Sheet, and seeing the number of bolded players.
There are more than are shown, because only those with 400+ GP are shown, as well as players with less than 400 but who have scored 10 GP or more for the week.
The monthly will be a bit more accurate, but there will still be more than is shown on the Scandal Sheet.
Another quick check can be found here:
http://ladderslasher.d2jsp.org/index.php?a=3&h=0&t=5It is current to within 1 hour of the time frame covered (1 day, 1 week, 1 month), but is limited to showing 50 players