Re: How to tell which GPU is used to render?
If you have physically separate X screens without Xinerama, they're named ":0.0", ":0.1", and ":0.2". Perhaps try experimenting with those DISPLAY values? But, if it's displaying on the screen physically connected to the faster GPU, it's extremely likely it's rendering there. Xinerama might complicate things; I forget exactly how that works in OpenGL.