The best way is to use a differential probe, the signal measured at this time is the most real and objective; if there is no differential probe, you can use two differential probes to connect to the two channels of the oscilloscope (such as Ch1, Ch2), and then use mathematics Calculate, get the waveform of ch1-ch2 and analyze it. At this time, try to keep the two probes exactly the same. The Vertical scale (how many volts per division) of the two channels of the oscilloscope is set the same, otherwise, the error will be large.