I don't think you can know when the yeast has hit high krausen, since the constant stirring prevents a krausen from forming, but with a starter, you don't really need to know....

The idea behind pitching at high krausen is to pitch actively fermenting yeast. With good yeast stock, after about 18-24h your starter will be actively fermenting, and will have gone through several reproduction cycles, so the cell count will have been bumped up and viable cells will be in solution. So, you can then pitch, knowing that viability is good and that the yeast are actively fermenting. 

Before pitching, turn off the stirplate and leave the yeast to settle for 30-45 mins. You might see the start of a krausen ring form, and the dead or inactive yeast cells and trub will sink to the bottom, leaving the active yeast in suspension in the wort, which you pitch, leaving the trub behind.