Wikipedia has an informative article (https://en.wikipedia.org/wiki/CPU_cache); I don't know how much is correct, but it seemed reasonable.  From that article, if I understood correctly, checking the contents of a cache always happens, so it's not an extra penalty.  The penalty is in requesting from a higher-level cache (a request may not go to main memory).  You might mention that these statements are also a simplification of the real situation.