How do I know if I am a PLUS member?

There are two ways: 

The first is to go to your profile and you will see that your profile will have an animated background that displays the badges of your choosing (more about that here). 

The second is to check your settings, which can be reached by clicking the settings button in the top right of your profile and then clicking the "Account Settings" tab. There will be a header that reads "SongPop Plus Status" and will indicate if you are a subscriber or not, and if you are will show how much time you have left until it expires/auto-renews.

In addition, your display name on leaderboards and on other player's list of opponents will be a bolded purple colour.