Yup, there's a product ID stamped into every gp32 via the e2. Not exactly easy to read mind, as it was the basis for part of the copy protection used on games, but it's certainly not impossible.
I think you can read it via the i2c bus, so get learning how that works, and then probe for it's address. Don't forget the sound chip (well, dac) is on there too though, but you should be able to find the address for that by looking through mirko's sdk sources.