Verifying doubles are a stricter alternative to normal doubles that provide guarantees about what is being verified. When using verifying doubles, RSpec will check that the methods being stubbed are actually present on the underlying object if it is available. Prefer using verifying doubles over normal doubles.
No checking will happen if the underlying object or class is not defined, but when run with it present (either as a full spec run or by explicitly preloading collaborators) a failure will be triggered if an invalid method is being stubbed or a method is called with an invalid number of arguments.
This dual approach allows you to move very quickly and test components in isolation, while giving you confidence that your doubles are not a complete fiction. Testing in isolation is optional but recommend for classes that do not depend on third-party components.