Thanks for a great question and, yes, it is a VERY tricky one to answer. There are several ways to go about it. First, the Bureau of Labor Statistics publishes what's called the Occupational Outlook Handbook which you can find here: http://www.bls.gov/ooh/ It provides tremendous insights on salary, educational requirements, etc. Another option would be look up the salary information for your public counterpart. For example, let's say you want to work as an auditor. You can then look up public employee salaries in your state to see what auditors make in your state on average (which may be higher or lower than the average salaries made in the private sector).
I'll admit upfront that this is not a perfect system because salary information differs based on a variety of factors, e.g. public vs private employees, size of the organization, education/experience requirements, etc. But it will give you a good idea of where to start.
Another good idea would be to seek out internship and/or volunteer opportunities for the career you are interested in. Volunteermatch.org is a great place to get started. Many people tend to think of volunteer opportunities as working at animal shelters and picking-up litter, but if you'll quickly discover that volunteer opportunities exist for everything from office support to marketing and on the list goes. Doing this allows you to not only develop direct experience, but it gives you an opportunity to network with people and build relationships. Through these relationships you'll be able to get an understanding of salary information simply by asking.
Eventually you'll have a pretty good idea of what the particular job you want will pay. From there you can establish a minimum salary in your mind and then extend it to a certain range. Many people have a minimum "plus 5" or plus "10" which means that if your minimum salary expectation is $45,000 your "plus 5" would be $50,000. When an employer asks you about your salary expectations you can give them your range and simply say "I'm looking for something in the $45 to $50,000" range (or whatever your plus range is) and then simply ask if that will be acceptable. If so, then boom... you've cleared that hurdle. If it's not acceptable to them then it means they are either unwilling to match your salary or they simply don't have the capability... either way it's still a win for you.
Two additional notes: It's important to think about compensation as entire package deal and not just based on salary. Is the job that pays $52,000 a year that has two weeks of vacation and no matching 401k a better offer than the company that pays $48,000 per year, but has 4 weeks of vacation beginning the first year and a 401k match? That's something you'll need to decide for yourself, but I do caution everyone to avoid becoming fixed on salary alone because there are a lot of other benefits and perks that may, in the end, be more important to you.
My final note is for you to check out the book called "Get Paid What You're Worth: The Expert Negotiators' Guide to Salary and Compensation" by Gregory Northcraft and Robin Pinkley. It's a few years old, but it still provides tremendous insight on how to make the most of negotiations to get a deal that is uniquely suited to you.
Best of luck!