Sometimes it is possible to obtain an accurate estimate of how close a pass was by comparison with the size of a known object.
This example shows a still taken from a video of a Mercedes overtaking one cyclist, with another cyclist (in yellow) in front.
A number of number plate checking sites (e.g. https://www.checkcardetails.co.uk/number-plate-check) allow you to check the model and colour of vehicles as well as whether they are insured and have an MOT. In this case 'FV 61 DZO' returns 'Mercedes-Benz E250 Bluef-CY Sport CDI A, Silver Diesel'.
Using Google you can look up the dimensions of the vehicle. In this case Mercedes E250 has a width of 2.015 metres (blue arrow in photo). The yellow cyclist was 0.38 metres wide (yellow rectangle in photo).
By measuring the width in pixels of these arrows in the image, and using ratios, we can determine that the gap between the rear of the car and the yellow cyclist was 0.81 metres. Allowing for the 15 cm extension of the wing mirror - this means that the gap between the car's mirror and the cyclist was about 0.66 metres.
This particular incident resulted in a NIP.