I think the idea is when making the 3 stops over exposed image, there should be a coin in the image...if it just appears in the contact print, then you have got detail in zone 8, which is by definition what should be seen in zone 8, hints of highlight detail.
Cheaper than a densitometer, although requiring more frames of film, shoot zones Z1 through to Z8 and then get a Darkroom Automation meter to measure the light intensity under the enlarger of each frame. Also shoot a reference blank frame for measuring Base+Fog. You dont need to even put the negative in the enlarger, just lay it over the top of the meter. Calculate contrast index by subtracting Z8 from Z2 reading, and then dividing by 6.
I've just done this for some TMAX 400: I got light readings of 8.91, 8.22 (base + fog), 8.04, 7.75, 7.53, 7.10, 6.63, 6.13, 5.63, 5.17, 4.42, 4.04. To interpret this for CI, I've got a change of 7.75-5.17 for Z2 vs Z8, so a 2.58 stop change in light intensity for a 6 stop change in exposure. This means a CI of 0.43. This means I am underdeveloping my film. Interestingly, and tallying with the theory, my Z1 is nearly still ok, at 0.16 stops over B+F.
Depending on the aperture range of the lens, you can be lucky and get the light adjustment by altering aperture and leaving shutter on one setting, to avoid discrepencies brought about by changing shutter speeds (this works on my Pentax M 50/1.4, which runs from f22 to f1.4, just meter at f5.6 for Z5. You could just shoot a blank frame and then a few zones, but by shooting the whole lot of zones, you can ascertain whether your EI is wrong, all at the same time.