The short answer is redline.
The slightly longer answer is to determine by feel where (in terms of RPM) the power drops off. Hold a gear as the revs build and you will feel a point where you quit pulling stronger and the power actually drops off a bit. Your max accel will happen if you shift just as the revs pass the max power point.
Of course, if you were to look at dyno graphs of your engine, you'd see what you're feeling. But, personally, I think these things are best learned by feel than by some form of rote memorization.