shader_current();
Returns: Real (ID value of the shader, or -1 if no shader)
This function will return the index ID value of the shader
currently being used for rendering, or it will return -1 if no
shader is being used.
if shader_current() == -1
{
shader_set(sh_warp)
}
The above code will check to see what the current shader is and if it returns -1 (no shader being used) then a shader is set.