Facebook revealed earlier that they would let users know if their data was in accessed by Cambridge Analytica or any other company. In keeping with this promise, they have done so, and here is how you will know.
It is simple. Log onto Facebook, and head to your news feed. If your data was improperly accessed, you will have a notification at the top of your feed telling you who has your data. If you have no notification, it means your data is secure (for now!).