The Cake:

White cake is traditional, but not necessary anymore. Any flavor is good, be sure and taste samples!

If you're putting flowers on the cake, make sure there are no semi-poisonous ones. Some flower petals can make people sick
Make sure that the cake can be delivered at a time convenient to you.

Know how many people the cake will serve.

There should be a contract with the baker, which specifies: shape, size, flavor, color, decorations, delivery day, delivery time, delivery place and price.
Who’s cutting the and serving the cake? Do they know what they’re doing? Especially hotels and non-wedding-specific venue employees may not have experience in SIZING the portions. If they cut the pieces too large, they’ll run out of cake. If you don’t have experienced wedding servers cutting and serving your cake, specify small portions. You can also do the “cake switch” and have a spare cheap cake as back-up.