Dissolve the whole heap in water? Or should I read the article to learn this isn’t a physics question ;-) ?

Yeah, I think that's the right answer. Dissolve it in water and run it through a smallish filter. Other impurities in the salt can clog the filter sometimes.

So close, it was in fact a philosophy question ..

https://plato.stanford.edu/entries/sorites-paradox/

"How many grains of sand change a heap of salt into a pile of manure"