Normally I just compare the results of a cable in question to a known good and determine if the values are too far out of spec. But surely there are faster ways to do that? I have been checking the videos online to look to see about diagnosing a cable. I am aware there is the use of the LogMAG set up to identify a break, as well as the use of the TDR. I think these two technically compliment, though the LogMAG way is only option for older hardware.

I have a NanoVNA-F v2 and it has TDR.
Though the device says the a 20ft cable is 39 Meters, which makes no sense. So I must be doing something different from the videos.

I have a NanoVNA-H4, and it has a TDR function but it's marked as Cable S11, not TDR.
So far unlike the F V2 which gives an estimate of length, the H4 gives a cable loss.
I see there is a function to add a cable length value and this generates a % which is a multiple of the expected value relative to what was measured.
After re enabling everything I was able to get it to read a singular length, though its reading 88.3 meters with and without the cable.

Both devices I have tried set up the LogMAG method. Never have I been able to get it to match the online demonstration. Either the cables I am testing are that bad or I have over looked something.
The video, he disabled all but S11 logmag on screen, though it Said CH1 LogMag. I am not sure what version of firmware he has, my H4 says M1-M4 if multiple markers are active and S11 if it is a Trace.
He seemed to be running 50kh to 5Mhz, and M12 is set to 5Mhz. He did a generic tuning, Open, Short, Dummy, and Thru. However his thru was separate from the other calibrations. I did as he did but though per the DG8 that to calibrate a lead, the Dummy, Short and Open had to be on the ends of the the leads in order to have them cancel out. Like solving a quadratic Equation.
So in the end I have not been able to get the F v2 or the H4 to behave the same or to get them to read the same values as the video.

So the question is should I try to get the LogMAG method working, or focus on the TDR functions?

Per a previous topic question I am generating a couple charts that compare the hardware and firmware options. Work in Progress.

You must enter the velocity factor (usually 60%-90% based on the particular cable type) to make any measure of the cable length using either method, and leave the other end of the cable 'open'. If you don't supply the velocity factor of your cable, the instrument has no way to calculate the length, and will give a length for whatever default velocity factor it has set.
I almost always use the "Measure -> Cable(S11)" feature of the Nanovna-H4, because it is very convenient to use and gives reasonably accurate results.
And note that this is NOT a TDR function re-labeled.
You can alternately use the TDR method on the Nanovna-H4 by turning on "Display-Transform", which then shows the TDR response as calculated using an FFT from the frequency scan data. There are pros and cons for each method. The TDR method works well, and can also locate partial faults along a cable, but is a bit trickier to set up.
The "Measure->Cable(S11)" feature requires only that you a) leave the other end of the cable 'open' to get a full reflection, and b) that you set the start frequency low and the stop freq such that the trace on the smith chart traces at least one half-circle on the display. Then it displays a measure of the cable impedance, the cable length (based on the velocity factor you enter), and the cable loss at the frequency of your marker. If you know the exact cable length, you can enter that and it will instead calculate the velocity factor. This feature measures the 1/4 wave resonant frequency corresponding to the cable length, and uses that for length calculations, and the S11 return loss to calculate the loss in the cable. If you have versions of the firmware before 1.2.37, the loss is a simple calculation of 1/2 the return loss (since the measured loss is for two ways on the cable). So this is your 'logmag' measurement (at least a single-sided one). The newest versions of the firmware make a more complex quadratic calculation that is more accurate at VHF/UHF frequencies.
Good Luck.