Most iPod or iPhone adapters will tell you on the screen of the device after plugging it in, whether it's compatible.
Also compare the miliampres rating between the two. If the 12volt adapter is rated lower than the original charger, you should be fine.
Some cabling adapters, especially those for vehicles will change the ground pinouts of the adapters making it not allow certain audio functions of the iPhone or iPad.
This is because of a limitation of resistive wiring on I believe pin 11, to allow for audio to be sent out of the device as a line out instead of using internal speaker.
So although it may work as a charger, it may inhibit certain audio functions untill you remove the adapter.
