query capabilities

in GLview I saw that they can query what tex_env_combine functions are possible on the system.
testing the engine on multiple systems so far I saw that a sis640 (nightmare !!) based graphic card claims that it supports tex_env_combine but then had to find out it doesnt support interpolate function

how do they query in GLview what modes for tex_env_combine are allowed ?
do they just test all and check with glerror ?

I looked up ext_tex_env paper on sgi but couldnt find anything how one could get with glGet