This is a pretty complicated question, with a number of answers. First of all, I think you should be at the point where you are already making money from hobby photography - either through small product shoots, architectural stuff from craigslist, shooting friends small weddings, etc.
Second, if you already have a job, and you are looking to replace it, then its pretty easy - do you know that you could be making more money with photography, but your job is in the way? Can you project your income from photography, plus the additional benefit of doing something you like, putting you past the income of your current job? If so, its time to think about it.
Third, have you subjected your work to a series of peer reviews? This may be the most important step, in my opinion. Make sure that you have a good resource of pro photogs, and see what they think about your work. Take their advice and improve certain aspects that they recommend. Keep in mind that while a number of people out there shoot as wedding photographers, baby photographers, and class photographers, they probably wouldn't really fit into the skill level of a true "professional". I guess what I mean to say is that you can make money, or you can make money and take good shots.
Of course this is all said by a photog thats made less than a total of $1k!