If you want to test a product to see how good it works, without doing it on your wind shield first, test it on a old CD or DVD. They're made from polycarbonate, and someone usually has a scratched one around to experiment on.
If it works, you resurrect a disk, and find a product for use on your shield, win-win. If not, then polish the disk with some toothpaste, followed by plastic polish or finishing compound to revive it.