This article is from the Amiga CD32 FAQ, by Stuart Tomlinson with numerous contributions by others.
A: If you have a CD+G disc, you'll be able to tell immediately - the music player will disappear and the graphics will appear as soon as you press play.
Use the up and down directions to change language and select to return to the music player. The rest of the controls work as normal. Once you're back at the player, a "CD+G" button will appear in the bottom left corner - hit select on that to return to the graphics.