Ask HN: questions to ask freelance designer

I've found a designer who has a portfolio I like. We've agreed on a detailed spec for the design work and agreed on the price and payment terms (50% up front, 50% on delivery). The deliverables are html and css files along with any required graphics.

This is the first time I've hired anyone. Are there any things I should make sure of before I pay him or are there any things I should insist on with regard to our agreement?