It's easy enough to figure out. Render the same glyphs twice with FreeType, once black on white and once white on black. If you then invert one of the images they should match if you're rendering into a linear colorspace. If they're different then gamma correction is being applied.
Font textures that are rasterized with proper gamma will only look correct for the colours they were rendered for. So if you create a texture with all your glyphs on it that's white on black, and then try to use that to render text that is black (or any dark colour) on white (or any light colour) you'll end up with really ragged looking text. You're effectively applying the inverse of the desired gamma curve.
Supersampling the fonts and then reducing with a simple box filter will have the side-effect of forcing a "linear" result and will give you better results than bad gamma, but not as good as proper gamma.
You should probably familiarize yourself with how gamma/sRGB works before trying to understand gamma correct antialiasing. Try this article:
The Importance of Being Linear or read
some of the stuff on Anti-Grain for something more font specific.