If you want to quickly verify that a potential employer is willing to pay a 'competitive' salary, why don't you take the initiative and tell them not to consider you if the position pays less than $x?
Honestly I just haven't had to. The job market hasn't gotten that bad and there are enough jobs where I can pre-filter based on salary(recruiter/job board/etc) that I've haven't had to write specific e-mails asking for compensation.
I also imagine the response rate would be low. Some wouldn't respond, some would respond with no, and some would respond it depends. And of the ones that responded yes it could mean yes, but it could also mean they are only willing to pay their purple unicorn candidate that amount.