The counter application is quite simple: the whole state had just two pieces of data (the current counter value and the number of clicks so far) so it's easy to follow what happens during a test run; see the next screenshot:
A sample run of the counter application, but logging all actions with redux-logger
You can easily follow the test run, and you'll be able to see when we clicked each of the buttons which action was dispatched and the successive values of the store—if there were any problems with the reducer's logic, you'd probably find them easy to detect, given all the information that appears onscreen.