one way is just go to the screen where you can change screen resolutions. It should show all the resolutions the card supports. However your monitor will have its own limitations so everything you see may not work.
I have an NVIDIA GeForce GTX 780 but the official website ( http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-780/specifications ) says:
Maximum Digital Resolution: 4096x2160
Maximum VGA Resolution: 2048x1536
However it also says,"Note: The below specifications represent this GPU as incorporated into NVIDIA's reference graphics card design. Graphics card specifications may vary by Add-in-card manufacturer. Please refer to the Add-in-card manufacturers' website for actual shipping specifications.
I do not know if my card is an Add-in-card or not because my pc was custom made and I do not think I have the documentation for it anymore. Is there a way for me to find out? I have looked for programs on download.com like PC Wizard 2013 that detect hardware specifications but it was not able to tell me this.

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