[R] R2 in multilevel modelling

[klair87 at libero.it]

Goodmorning everybody,
i'm an italian statistician and i'm using R for research.

Could someone tell me some indices to see the goodness of fit in multilevel 
modelling?
I'm using the lmer function, and I want to know if my model fit well my 
data.  
I actually want to justify the use of multilevel model instead the classical 
one.

Hope someone can help me. 
Thank you.

Greetings 
Chiara