Graphic adapters in DOS mode.

I am using an old DOS based program that uses grahics in VGA (640x480) mode.
The program uses a TSR to redirect any graphics commands within the program
to the hardware registers of the graphic adapter. So far so good, however
when a text is displayed while in this VGA graphics mode, some graphic cards
display garbage while other cards display readable text. In some cases I
have to preload text drivers, like TsrFont.com, Fnt8x14.com or others to
display true text in thge graphics mode. How do I determine wether or not a
graphics card displays true text in the VGA garphis mode? This thing puzzles
me since the first version of W95.