This function will do a check to see if the chosen target platform supports shaders, returning true if they do, and false if they do not. It is important to note that on Android, if the project does not have any shader resources defined, then the function will always return false, regardless of whether the device supports shaders or not.
global.GFX = false;
if shader_is_compiled(sh_glass) && shader_is_compiled(sh_warp)
global.GFX = true;
The above code will set a global variable to false, and then if the platform supports shaders and both the shaders being checked have compiled correctly, it will be set to true.