It's usually fairly easy to spot someone who is dissatisfied in a restaurant environment.  Watch the areas where employees think they are out of sight.  Does their body language change when they leave the area where customers can see them?  Do they volunteer to help each other or do they only work independently?  Do the employees seem to get along and have a good time or are they just trying to make it to the end of the shift?  A dissaastified employee might smile for customers but they aren't going to maintain the perky personality in the back areas.  They might even be heard complaining and grumbling as they enter the kitchen or drink stations.  It's usually possible to stop the difference between a waitress who is keeping things in order and one who is rushing to leave as soon as possible.

Physical appearance is important, but not as important as the way waiters/waitresses greet their customers and respond to them. Some employees just go through the motions, but the ones who smile and seem genuinely happy with their work are usually easy to spot.

I would look at what they are doing when not waiting on customers. Are they "cleaning or leaning?" Many employees, when not happy, tend to simply stand around and not talk to anyone. If the employees are talking to each other, staying busy, and have genuine smiles on their faces, one can assume that they are generally happy.

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.

Watch how they interact with each other when they think nobody is watching. Do they seem happy, courteous, professional, even friendly in their interactions with each other, or do they find corners to whisper conspiratorily to each other? Another way is to listen to what they say when things go wrong. Do they blame themselves, the kitchen, managerment? In most healthy work environments I've been in, people take responsibility for the business as a whole.

In searching for information regarding employee satisfaction, I would want to learn if employees felt they were being provided adequate training for their specific jobs; if they felt they were being treated fairly in terms of scheduling and assignment of duties; if they were satisfied with the handling of wages and benefits; and so on. In the specific situation of a restaurant, I would ask if the employees felt the atmosphere and provisions contributed to satisfactory tips.

I would look at the way the employees look physically.  Do they seem to care about how neat they look?  I'd look at how happy they seem.  Do they project a welcoming attitude to patrons?  These would be the clearest indication of employee satisfaction.

I could ask questions like "are you happy in this job" but I'd trust my eyes more than the answers they'd give me since the employees might think I'd turn them in for giving bad answers.