Direct3D or OpenGL???

I dont know how to find out if a game is direct3D or openGL. I am testing with anti aliasing and anisotropic filtering but it seems you have to turn it on for most games in the videocard control panel. With ut2k4 it says direct3d in the menu, but what about other games such as GTA vice city and avp2?

 

Direct3d seems to be the most videly used and accepted since this is the one being pushed by microsoft and directX. Open GL is still supported in most games for compatibility with older grphics cards but i recommend Direct3D