Is there a way to measure and verify the actual voltage of the EFx32xG1 internal 1.25 V or 2.5 V ADC references, such as by outputting the reference voltage to a pin?
The internal 2.5 V and 1.25 V references are based upon the internal bandgap reference (BGR), and the full scale reference value (i.e. 2.5V or 1.25V) is achieved through manipulation of input signal attenuation settings. Thus, there is not a 2.5 V reference value that could even be measured per se. Additionally, the internal reference sources can not be routed to a GPIO pin.
Additionally, because the ADC internal reference source in the EFx32xG1 device achieves different VFS values through input signal attenuation factors/dividers, the voltage of 2.5 V (or 1.25 V, or 5 V) does not actually exist in the device as a node voltage. Because of this, we do not specify any absolute voltages for the ADC reference sources, but instead specify a gain error for the ADC module (see "Gain error in ADC," EFR32xG1 datasheet, Table 4.39, section 4.1.16, page 75), which is -0.2% typical and 5% maximum. Thus, this specification can be interpreted to mean that the VFS when using the 2.5V internal reference option will fall within the range [2.35 V, 2.65 V] (worst case). Typical variation will be much tighter ([2.495 V, 2.505 V]). The ~0.833 V BGR reference from which the internal VFS levels are derived is calibrated for each device in production.
If you wish to monitor your ADC reference voltage directly, you can use an external reference by setting ADCn_SINGLECTRL.REF (or ADCn_SCANCTRL.REF) = EXTSINGLE (0x4) and connecting a reference source to the ADCn_EXTP reference pin (for single ended reference).