Have a bunch of different systems for testing, I can't imagine a way of getting around that.
Optimally you should have a card from every graphics generation.
- GeForce 7000 series
- GeForce 8000 series / 9000 series / 250
- GeForce 260 / 275 / 280 / 285 / 295
- GeForce 400 series / 500 series
- Radeon X1000 series
- Radeon HD 2000 series
- Radeon HD 3000 series
- Radeon HD 4000 series
- Radeon HD 5000 series / HD 6000 series
- Intel GMA X series
- Intel HD series
If you want a really thorough testing you can multiboot all these machines with a bunch of different operating system and driver configurations, but for a more manageable number of tests you can do multiple operating systems and drivers on just one machine each.
Apart from all this, why don't you diversify your dev machines? Maybe you'll also discover more subtle bugs earlier.
Even if you can find a way of testing this exact subtlety there is no way you'll be able to test all the things you should without getting at least one nVidia card.