Testing the Service layer
It is tempting to allow the testing of the Service layer to be done via tests around the calling code, such as Apex controller tests. However, depending solely on this type of testing leaves the Service layer logic open to other use cases that may not strictly be covered by the controller logic. For example, a certain Apex controller will only pass in a single record and not multiple ones. Make sure to develop specific Apex tests against the Service layer as the functionality is developed.
Mocking the Service layer
Sometimes, the data setup requirements of the Service layer are such that it makes writing Apex tests for controllers or other callers, such as Batch Apex, quite complex and thus expensive, not only in server time for the test to run (due to data setup for each test method) but also in developer time while preparing a full set of test data.
While you still need to test the full stack of your code, there is an alternative approach called...