For the most part, if your employer controls the time, place and manner of your work, you are an employee. For instance, if your employer can discipline you, tell you how to do your work, have to approve vacation time, and you cannot hire an assistant if you want, these are also signs that you are an employee.
There's a handy form the IRS has, the SS-8, that you can fill out and the IRS will do the work for you of determining whether you are a contract employee or independent contractor.
If you are a contract employee and not an independent contractor, then you are covered by anti-discrimination laws (assuming the other statutory requirements are met).
Just remember that there may be a question as to who is liable - your actual employer (like a temp agency) or the company for whom you provide services. If you haven't reported it the discrimination to the employer who writes your checks, it is likely a good idea to do so. If the company that you are providing services for controls your work and you enough, they may be a joint employer and also be liable.