Is the company full time? How quickly did the company get back to you after initial contact? Many companies have started a part time business and if they have you want to make sure that they are still responsive. Make sure that the company is fully licensed and insured. Many venues require the latter and require a certificate to confirm this. Also the quality of website that the company has is a pretty good indicator of how serious they are about their business.
How nice are the prints? A photo booth company that spent the extra time ensuring that they are producing high quality prints is worth looking at. How nice does the graphic and text look on the strip? How quickly do the prints come out? This is a very important aspect of your photo booth experience and is often overlooked. Ask to see samples from different companies.