You'll come to know that yourself!
When POP is activated, you will see a "New Features!" link on the top right of your GMail account like this:
(Incidentaly I got this message today itself!)
Then to enable POP in your Gmail account, follow these steps:
Click 'Settings' at the top of any Gmail page.
Click 'Forwarding and POP' in the orange 'Mail Settings' box.

Select 'Enable POP for all mail' or 'Enable POP for mail that arrives from now on.'
Choose the action you would like your Gmail messages to take after they are accessed with POP.
Configure your POP client, and click 'Save Changes.'