In order of importance
1. box position: A dedicated foil board will have the Tuttle box located in the correct position relative to the foot straps for foiling. Some boards will have a Tuttle box and slot boxes so any foil can be fitted.

2. Tail width: Foil boards have wide tails which enable you to more easily balance your foil, especially when heading up wind.

3. flat deck: Foil boards have very flat decks. It is far more comfortable to stand near the edge of your board if the deck is flat. On a board designed for fin use the deck is curved towards the edges. It is harder to stand near the edge in an upright foiling stance.