Once you hop on the Internet, you can do a search for wedding bands in your area. You can even narrow your search by theme, such as country, rock, swing, jazz, Top 40 and so on. Chances are, your best bet is to find a good band that can play a variety of music, which will help cater to a wide age range at your big event. If the wedding band has a really good website, then chances are they will have photographs of how they look when they play live. This will give you some type of idea of the feel of their stage presence so that you can get more of a vision of how they will look during your reception.
You are going to want to be sure that you either get a chance to hear a CD of them playing live or actually view them playing at a venue. This will give you a good feel of their expertise and you will also be able to gage the crowd reaction to their live set.
It is also a great idea to ask around to see if you can get any recommendations on the wedding bands that you are researching. Often times, there is no better reference than a bride who already used the same wedding band at their reception.