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, in many ways, 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. You can pitch, knowing that viability is good and that the yeast are actively fermenting, even though you don't have any visible signs of fermentation. (A quick test is to take the flask off the stirplate and swirl it - you should see lots of CO2 bubbles coming out of solution.)

Before pitching, you may want to turn off the stirplate and leave the yeast to settle for 30-45 mins. Then you might see the start of a krausen ring form. Also, 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 and lazy yeast behind.