The technique below should work if your BIOS is not too old. It uses
 three functions from INT 10, the BIOS video interrupt. (If you're using
 a Borland language, you may not have to do this the hard way. Look for a
 function called DetectGraph or something similar.)

 Set AX=1200, BL=32 and call INT 10. If AL returns 12, you have a VGA. If
 not, set AH=12, BL=10 and call INT 10 again. If BL returns 0,1,2,3, you
 have an EGA with 64,128,192,256K memory. If not, set AH=0F and call INT
 10 a third time. If AL is 7, you have an MDA (original monochrome
 adapter) or Hercules; if not, you have a CGA.

 This worked when tested with a VGA, but I had no other adapter types to
 test it with.