"Observo" is Eugene Burger's wonderful "observation test" routine created in 2000. His original instructions, plus many extras (including a video performance), is included in Eugene Burger: Final Secrets. 

The special card for this effect is difficult to find, so I have had a very high quality version of it made on Bicycle Rider Back stock. This package includes a full set of the cards required to perform "Observo," plus an extra special card for your future use.

Please note: no instructions are included, but they are fully provided in Chapter 14 of Eugene Burger: Final Secrets.