Producers know they are reaching the target audience in Call of Duty Black Ops because they look at the measured retail sales and the online gaming log in details. If someone is playing a video game online the information would be recorded. For example EA uses an online multi-player code system to count the amount of people who have bought the game brand new and it uses the server to count the amount of people playing online.
Producers know they are reaching the target audience in Facebook because of the number of people that sign, people that create an accounts and they have peoples personal information on the dater base. It targets audiences in way that it a makes it look more visually appealing and suitable to the appropriate age groups.
Producers know they are reaching the BBC iPlayer target audience because they can track what TV shows are being watched at any time, the types of programs being watched will give the producers a good idea of the demographic of their audience and therefore this will allow them to see if they have reached their target audience