There's a fancy new link in your profile page that shows when you're in the top X% of some bracket - week, month, quarter, year, and all time. Though, I haven't seen week and year myself.
What determines which bracket you're in, though? I originally figured it would be the highest bracket, but the #1 and #3 users on Gaming are shown as
top 1% overall, while player 2 only is shown as
top 1% this quarter.
It's not whichever bracket you are highest ranked in, either (as player 2 happens to be the #1 for this quarter). Fabian is listed as
top #1 this quarter, but his quarter rank is 4th, which is less than his month rank of 3rd.