What, if anything, is done to enforce laws regarding what employers can't fire employees over?

So say I have a sexist asshole for a boss, and he decides to fire me. Can’t he pretty much just say that my work wasn’t good enough and get away with being a sexist asshole? How exactly is this supposed to be enforced?

On a related note, I have a question about the clause that says that you can’t discriminate against someone with a disability. Is this only true if the disability doesn’t get in the way of performing the job? Say I need to take more sick days than your average person, does it then become okay to discriminate against me?