How to use the oscilloscope to measure the maximum value of one modulated waveform, which carrier frequency is dozens of KHz, and the frequency of the modulated wave is power frequency ?

The power frequency may be low frequency, say 50Hz/60Hz, and carrier wave is dozens of KHz, the period of one power frequency is about 20ms, if to observe a signal with period of 20ms, then the duration of oscilloscope acquisition window at least should be 2ms/div x 10div, and at the same time determine the sample rate of oscilloscope according to the carrier wave. At last, acquisition memory length which is required can be estimated, and judge whether it could meet the testing requirement.