assuming your device has a power input and return, you can measure the voltage across those inputs
to measure the current, you need to disconnect the return and put in a resistor and measure the voltage across the resistor as barry said
OR, if you have a clamp on current probe, either stand alone type or connect to scope type, use that, which is essentially the same as barry said
that will tell you the input power.
if you also measure the output power, you should be able to calculate dissipate energy and efficiency.
circuits with lots of I/O can be a bit more complicated, as each I/O moves energy around
for an answer more to your liking, please provide a sketch showing the inputs and outputs
maybe a photograph.
better information provided yields a better answer