Are you asking how one sets the strength criteria or how one checks that the criteria is met? That is, how do you select the 28 day compressive strength for a design or how do you check that the material used meets the selected 28 day strength?
Answer #1: You pick 3,000; 3,500; 4,000 etc. based on the size of the pile that each choice will result in vs. the cost of that size pile.
Answer #2: You take standard test cylinders of the concrete at the time of pouring the pile and test them to see if they meet the specification.