@starrynight - Misdiagnosis would be a very bad mistake to make. Most people think that lab results are completely definitive, so I can only imagine the fall out if a lab technician messed up on something like that.
I got to look at a few slides of atypical lymphocytes when I took Anatomy and Physiology. They were relatively easy to identify if you first knew what a lymphocyte was supposed to look like!
As the article said, the first clue is that an atypical lymphocyte is rather large. If you can identify that, then other irregularities will start jumping out at you.