It is ok to look at the cut-off frequency of the filter with an oscilloscope, but you need an excitation source to excite the filter. This excitation source can emit a sine wave with a certain frequency, which is greater than the cut-off frequency of the filter, or it can be a frequency scanner.

Connect the signal source to the input end of the filter, set the sine wave output with the amplitude of 1V, test the output signal of the filter with an oscilloscope, gradually increase the sine wave frequency of the signal source, and record the signal amplitude seen by the oscilloscope. When the amplitude measured by the oscilloscope is 0.7V, it is the bandwidth of the filter. When there is no signal on the oscilloscope or the amplitude is extremely small, the frequency of the signal source is its cut-off frequency.