Well if you have not gone out of your way to install the Nvidia one then you are using the nv or vesa look into the /var/log/Xorg.0.log or /var/log/XFree86.0.log file depending on which you use and it will tell you in the section like this.
Code:
(II) LoadModule: "nvidia"
(II) Loading /usr/lib/xorg/modules/drivers/nvidia_drv.o
(II) Module nvidia: vendor="NVIDIA Corporation"
compiled for 4.0.2, module version = 1.0.8762
Module class: X.Org Video Driver