Even if you don’t have a Rohde Schwarz oscilloscope, you can still enjoy their recent video about using an oscilloscope to measure power supply efficiency. Of course, you don’t have to have a scope to do this. You can use a voltmeter and an ammeter, but it is very straightforward if you have a four-channel scope with a pair of current probes.
Of course, if you can measure the voltage and the current at the input, you can calculate the input power. Then again, most scopes these days can do the math for you. Then, you make the same measurement and calculation at the output. If you know the input and output power, you can calculate a percentage or many scopes can do it for you now.
The video reminds you that modern power supplies are often very efficient, so you want precise measurements. Depending on your equipment’s capabilities, you might do better measuring the current with a precision meter or using a sense resistor in series and measuring the voltage, although, obviously, a current probe will be more convenient.
Of course, if you only have a two-channel scope, you could do the measurements sequentially, assuming the power supply is in a steady state or close to it. You could also do the measurement with one or more multimeters. After all, multimeters are pretty cheap, so dedicating four of them to this task wouldn’t be that prohibitive.