The way the employees act toward customers is a good clue. Some employees don't do a very good job of "putting on a happy face" when they don't like their job. Employees who seem to be in a genuinely good mood are probably at least fairly satisfied with their job.
Look at how the employees act as they speak to each other. Are they smiling and laughing, or do they appear to be stressed and unhappy.
Finally, how good of a job are they doing? Do they do the little things, or are they just trying to get finished as quickly as they can.