If you have installed a NVIDIA graphics driver, it ships with the nvidia-smi tool that can be used to list the GPUs on your system. More information can be found in my nvidia-smi cheatsheet.
If you do not have a NVIDIA graphics driver or for some reason it is not showing the GPU name correctly (for example it is older than your GPU), then the lspci tool can be used instead:
$ lspci | grep -i nvidia
The lspci program lists all the PCI devices on the system. If there is a NVIDIA graphics card, it should be listed as a VGA compatible controller along with its model name.
Instead of a model name, if you only see NVIDIA Corporation Device, then maybe you are using a card released recently. Update the PCI ID information on your computer and try the command again.
You can also lookup the Device ID directly in the NVIDIA device ID list.
More information can be found in my lspci cheatsheet.
Tried with: lspci 3.1.8 and Ubuntu 12.04.1 LTS