[R] Number of components in a mixture model

[kevin xie]

Dear all,

I'm fitting a set of length-of-stay data by a model of mixture of 
exponentials. I've been following the example on page 436 in MASS (5th Ed.). 
However, I have a couple of questions while following this example.

What if we don't know how many components there are in the model in advance. 
Is there any established method to determine the number of components from a 
set of data? I'm aware that the usual likelihood ratio test is not 
appropriate in this case due to the possibility that the ML could occur at 
the boundry of the parameter space.

Secondly, the example in MASS uses a Q-Q plot to informally assess GOF. I 
was wondering if there are some more formal statistical tests for this 
purpose.

I appologise for asking questions that are slightly out-of-topic.

Many thanks.

Kevin