1. In the Motherboard manual, if it's integrated graphics
2. Device Manager (devmgmt.msc)
3. DirectX Diagnostics (Dxdiag.exe)
4. Third-party tool such as EVEREST or Belarc Advisor
Also check "Debug - Video BIOS" section in the AIDA32 / Everest log. It might look like:
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C000:0180 S 6325/301 AGP True Color Graphics and Video Accelerator..... By
C000:01C0 tes Video Memory,...BIOS Version 1.05.00 ..Support VESA BIOS Ex
C000:0200 tension Ver 3.0..%..SiS.................Silicon Integrated Syste
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