How can you tell whether you are running a official Android program or one modified by your phone maker?
You can compare what your phone had with the emulator for your Android version. The emulator is a pristine Android, with no manufacturers modifications.

It is arguable whether Google-branded applications (which are not included in the emulator), are stock apps or not; but AFAIK Google-branded apps are usually not modified by manufacturers since they weren't open source and also Google-branded apps are updated regularly by the Market.