to really know you need some specialized equipment.
A microscope, and there is a special plate to put the yeast sample on, which puts a grid in the view for you, then you put on a measured amount of yeast slury - something like a drop, but it is very specific in size, like a micro liter (.001 ml) - ok, I probably have that size wrong. with it on the grid, you can count the cells. After geting a count (1500) in .001 ml, then you know you have 1500,000 per ml etc etc.
After that I've got now idea how to figure it from a batch of starter by eyeballing it. A brewer at work makes a judgement by the kerousan ... if he gets a high one for that style yeast, he repitches, if it is low, he buys new.