I have worked at a few different places in my life. I started out as a short cook for Pizza Hut. One of the things they teach you is to be friendly by sending the customers out of the restaurant with a friendly good bye. A lot of my coworkers would send people off with the same line all of the time "Have a good day" whether it was day or night. This is how you can tell if your server is just going through the motions.
I didn't go with the same rote memory line and tried to pay attention to the time of day and even what season it was. Especially if it was near Christmas, I would wish the the customer a Merry Christmas on their way out, which in itself could spark a debate, but I did it simply as a friendly gesture.
For those of you who work in the service industry or in retail, if you do send people off with the "Have a good day" line every time an easy way around all this is to just say "Have a goodn" which is especially effective the south. But at least try to be more personable, people know when you're faking it.