2 ways:

1) Online, your BLACKLIST shows all the numbers that have been blocked.  Remember that there are 2 types of numbers in the Blacklist: 1) Those that have been blocked by the Global Blacklist, and 2) those manually added by the users.  

2) You will hear 1-2 rings when the call is disconnected.  If you have a caller id phone, you will see the number in the display.