You need to find out the correct model and vendor of the graphics card on your system, to be able to install the appropriate drivers and get the hardware to function properly. Most modern linux distros can detect variety of graphics card, but do not always have the best driver for it.
So if you have an external graphics card like Nvidia or Ati, then you need to find the model name/number and then lookup further details online. Ofcourse, its easier if you have the hardware manual that came along when you purchased the computer. But here we shall be using commands to find out the same information.
The are only a few commands to learn. First one is lscpi and here is a quick example showing how to fetch details about graphics unit.
$ lspci -vnn | grep VGA -A 12
00:02.0 VGA compatible controller : Intel Corporation 82G35 Express Integrated Graphics Controller (rev 03) (prog-if 00 )
Subsystem: Intel Corporation Device
Flags: bus master, fast devsel, latency 0, IRQ 44
Memory at e0200000 (32-bit,...
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How to get information about graphics card (GPU) on Linux