How to distinguish between up-cut bit and down-cut bit quickly?
First, look at the right side of bit (it doesn’t matter whether the tip is facing up or down). Look at what direction the flute is heading as it goes around to the backside. If the flute is moving up as it cuts around the right side of the bit. You are looking at an up-cut bit.
For a second test, hold the bit in your hand with the tip pointed down and away from you. Turn the bit in a clockwise rotation and watch the reflection of any light on the bit. If the light moves up the bit as you turn it, you are holding an up-cut bit. The light will move downward on a down-cut bit.
What is the difference between upcut and downcut bits?
Down-cut spiral bits are great for dadoes, but go slow. Cross grain dadoes can be cut with a down-cut bit or an up-cut bit. The down-cut bit leaves a better surface, but you have to move more slowly to give the chips time to clear out the cut.
Up cut bit can leave a smooth bottom edge but a rough top edge.
Down-cut bit can leave a clean top edge but a rough bottom edge.