Lean Doughs

I just recently bought "Flour, Water, Salt, Yeast by Ken Forkish. The techniques and explanations in his book are great and I can wait to make some of the recipes. The only thing i am puzzled about is bulk fermentation. I wanted to ask what signs does everyone look for in determining when bulk fermentation is done, besides instruction in a recipe that give a time or say double or triple in size, can you do the poke test, I have heard people say to lightly slap the dough to get a feel of where its at. I would be curious to know how Ken has determined some bulk rises to be 5 hrs.