Unit Tests for Cache Class

Hi all, I'm just writing a small cache class to optimize my database connection. I want to use three different stages to store and receive my data as fast as possible. 1) fast get data from class variable if existing 2) mid get data from memcache if existing -> save result as class variable 3) slowest: sql query -> receive data from database -> save result to memcache -> save result as class variable How would you write (unit) tests for such a class? What is the best way to prove, that all data was stored correctly, even after data manipulations like insert/update/delete? I'm looking forward to your suggestions, Sebastian