How do you know that an oscilloscope is displaying the actual waveform?
For the fastest scopes, you don't have something faster that you can use to verify that the measurement is correct.


This is not true. We may not have time-domain that can display faster signals, or direct digital synthesis methods capable of generating signals at these frequencies, but we have been able to generate signals in the hundreds of gigahertz for decades. 

The LeCroy 100 GHz scope is, to my knowledge, still the only one with .1 THz bandwidth (though I've heard that might change in the next few years). I believe it was first demonstrated in 2014, and then released somewhere in 2015, but don't quote me on that. In any case, that bandwidth of real-time time-domain analysis has only become available within the last decade. 

But a quick google will show you people talking about sub-millimeter wavelength systems and physics (generally sub-millimeter is used to refer to signals with frequencies above 300 GHz) since the early 1900s. So, for over a century people have been working with these signals. 

Through using physical concepts to generate them, such as cavity resonators etc. we can generate signals that are very high frequency. Using non-linear devices, we can make mixers that operate at 1 THz now. So if we can generate this signal, and know it is a very pure sine we can input this into our new scope and start from there. 

When working at these frequencies, we very often don't work with the time-domain (so what an oscilloscope displays) but with the frequency domain (what a spectrum analyzer/network analyzer displays). In fact, I have been working with systems operating significantly above 100 GHz for a few years, but I have not used a scope with a bandwidth over 50 MHz in the last decade. 

The front-ends of those scopes tend to operate in a more frequency-domain way than a time domain way - they use mixers and power dividers to cut the input signal into a number of bands (in the case of the LeCroy, I believe it is 3 bands), and then mix each of these down to DC. Then we digitize all of those, and use very complex and smart DSP to stitch them all together. Using careful characterization of the system, we can allow the analog front-end to misbehave to some extent, as we can compensate for it in the DSP (provided it misbehaves in a very predictable and repeatable way).