Not enough downhaul:
Wrinkles from the mast to the leech are visible. The sail feels "heavy" and pulls very hard; you have a lot of pull on your back hand. The board isn't fast.

Not enough outhaul:
Part of the sail touches the boom, which should not happen with modern sail shapes. At full speed there is a lot of pressure on your back hand and the sail pulls very hard.

Too much downhaul:
The sail is far too flat and has no power. The leech is too loose. The board is not going as fast as it could. It feels like you have hit the brakes. The sail feels very light, but accelerates badly.

Too much outhaul:
As soon as the mast bends, there are wrinkles from the mast to the leech. The sail feels nervous at the top of its range and does not plane quickly. There is no power in the sail, even when you are overpowered.

Source: tips from North Sails and Arrows.