The jejunum is the longest part of the small intestines and is very curly and wavy. It is between the straighter duodenum and the short ileum towards the end of the small intestines. If you are unsure if you are looking at the jejunum, trying picking it up to see if it makes a circular shape due to its connection with the jejunal mesentary.

In this picture the jejunum is labeled. Everything is where it would be in sitting order in the cat without disturbance. Note the ileum which runs into the cecum.