Music Theory

I have a nub question; What does it means that a music was done in a determined scale? for example how you would recognize that pentatonic scale is found in Debussy's music?

It means that the notes in the song equal a certain scale.  Say the notes are CDEFGABC, for example, then the song's key is obviously C Minor and the determined scale is C Minor. It just means that the song isn't just a bunch of random notes, but is actually written using a certain scale.