Back to the original topic, I know that Sierra's games actually had stickers that said "256 colors" and not just "VGA" on the box. At leas they were clear about it. Even some of the screenshots on the back of the boxes would have subtext saying 256 colors (to differentiate from the 16-color alternative versions you could mail away for).
Thanks for the tip, looks like a few companies mentioned whether 256 colors were offered for VGA on the technical specifications sticker of the box. I notice Strategic Simulations, Inc. lists 256 colors for early VGA support in their titles (Buck Rodgers: Countdown to Doomsday, Dragon Strike) for example.
Sierra seems to be the worse offender, so many of their earlier games were only 16 colors, but stated VGA is supported on the box.
The best way to tell is to see how the game initializes the screen mode 😀
For 256 colour, they will use mode 13h, while for 16 colour, they use mode 0Dh (which is the EGA mode).
I always check by running DOSBox in the Visual Studio debugger, and putting a breakpoint inside the int 10h handler (ah=0 is the mode setting, with the requested mode in al).
Easier to just add console output for when video mode is set, if you can compile DOXBox.
That problem is that requires owning the games though, unless a game demo was been offered. Not all game versions are available online, even if I went with try before you buy route.