You can check if a username is already in use through the registration form and through the Friends system.
When you log into the game just click "new player" and get to the registration form.
If the nickname is not available then it will be highlighted in red and with a cross. As below:
If the nickname is available then it will be highlighted in green and with a tick. As below:
Don't forget that the nickname has to be chosen according to the Game Rules.
Customer support service by UserEcho