Ask to see his portfolio. You will either fall in love with his previous works, or will be indifferent. Choose with your heart, as this is going to be an emotional event to remember. You would want a person who can connect with your personal style and taste.
Ask about the equipment he will be using. Not all photographers are equal, when it comes to hardware they are armed with. Some big names will charge a lot extra, only because they have celebrities in their portfolio, but others could be cheaper or more expensive, depending on the cameras and lightening equipment they use. While the specs may not tell you all that much, you would probably prefer a person who has plenty of equipment to work with.
Price. In the end of the day, this is a deciding factor, because lets be honest, Washington DC is not a cheap place to find professionals at. You cannot book a photographer whose hourly rate is way beyond your means.