First of all, look at the nominal voltage value of the capacitor on the circuit board rectifier filter circuit (usually composed of four diodes or a rectifier bridge) to initially estimate how much the voltage of the circuit is, in general, the 5V power supply filter capacitor should be more than 10V, and the 3V power supply filter capacitor should be below 10V.
Secondly, model parameters can be marked according to the voltage regulator circuit components after the filter circuit, such as the voltage regulator module 7805 or 7905 are 5V voltage.
Finally, the 3V voltage can be determined according to whether there is a DC-DC step-down 3V module after the filter circuit.