How to to check if user has changed the password?
The easiest would be to do this using Rules. Following the next steps, should do the job.


Ensure you have enabled the "PHP filter" on the modules page. This is needed to be able to use PHP code inside Rules.   
Create a rule that reacts on the user event "After saving a new user account".
Add a condition of type "Execute custom PHP code" and use the following snippet: return $account_unchanged-&gt;pass != $account-&gt;pass;
Add your action(s) to send an email notification to the user or anything else that fits your purpose.