With a single probe EGT, you can KNOW that all cylinders are LOP IF the engine is running smoothly. If the F:A ratios are not even, the cylinders will be putting out different HPs and the engine will run rough. If it's smooth, then the F:A ratios are balanced.
As for Yr. Right's claim that LOP mixtures exit the combustion chamber still burning, in the usable mixture ranges, this is simply not true. For the mixture to be still burning as the exhaust valve opens, the mixture must be so lean as to be far too lean for practical use. We show several examples of this in the APS course. As one leans from peak toward LOP mixtures, the EGT falls… until the mixture becomes so lean that the EGT begins to rise with further leaning as the exhaust valve begins to open. This mixture range is so lean as to be very close to Idle Cutoff--unusable in operational conditions. This concern is a red herring.
There are a few mixture ranges where the escaping gasses actually COOL the exhaust valve. That is difficult for many to appreciate, but it has been measured.
There is MUCH refereed material in the literature. All of it agrees on the science. Everything John, Jabba, and I have been posting is completely supported in Taylor's definitive two-volume text on internal combustion as well as Haywood's excellent texts. FWIW, Taylor was the head of the Sloan Combustion Laboratories at MIT--no slacker, he. FWIW, I may be one of the few posting in this thread who has actually read both of Taylor's volumes. I highly recommend them if you have about six months to go through them. If you have another 4 months, study Haywood's text.
All of that is supported by all of the data collected by every engine manufacturer since the Wright Brothers.
This science is not arguable by anyone familiar with these multiple data sources. It is, of course, arguable by anyone with incomplete exposure to the science.
Old Wives' Tales in aviation like we are seeing posted in this thread are like snakes--they take a lot of killin'.