You can easily differentiate between the two organs. There is stratified squamous epithelium on the esophageal side and simple columnar epithelium on the stomach side. Note the absence of goblet cells in the stomach. In some species this abrupt change is located quite a bit deeper than it is in others. The transition point between the two is what the gross anatomist calls the margo plicatus inside the anatomic stomach.
Horses are an example of a species in which the transition is fairly deep, and so are rats.
To see the esophageal-gastric junction, click here. Once inside the actual stomach, you will find that the mucosal epithelium of the digestive tract differs in appearance and in function in different regions. Using slide 133, you should be able to see that the lumen of the stomach is lined with an ordinary looking simple columnar epithelium. Small openings into the underlying regions of the mucosal tunic are found at the bottom of these pits. If you took out your Swiss Army Knife and started excavations into the lamina propria and the regions beneath the pits, you'd find the actual secretory structures, the various types of gastric mucosal glands .