Oesophagus (100X)

How can one find out that this section is taken from the middle third of the esophagus?

The answer is that you'll find both smooth- and skeletal muscle tissue in the tunica muscularis (= muscular layer). Magnify to take a closer look at the muscle tissue; smooth- and skeletal muscle tissue

The upper third of the esophagus has only skeletal muscle, the lower third has only smooth muscle, while the middle third has both types.