No, not kidding.
I'm talking about the approach lighting, Icarus, which are very easy to recognise and judge.
Seeing as though they extend out to a known distance from the end of the runway (and depending on the type of approach lighting, the amount of perpendicular rows which are spaced at 150m), you can judge distance based off this. Look at ERSA AD to see the different types of approach lighting.
I expect that that is part of the purpose of the approach lighting.
For example if you are expecting that at the DA you will see all of the perpendicular rows of lights out your window but only seeing a couple, well you know instantly you do not have the required vis.
You wont be counting individual bloody bulbs on your fingers and toes!
The other 'measurements' people have mentioned here I presume are based off being on slope at the DA.
So, how do you do it Icarus? Got a good method to contribute?