The Liskov substitution principle (LSP) has a fancy definition:

Module A can be replaced by module B as long as B is a subtype of A.

Well-defined contracts heavily support this definition and help us reduce the coupling between modules. The following questions can help you figure out how well this principle is achieved:

• Are the modules interacting using abstractions or concrete implementations?

Here, the answer should be that the modules should not be interacting with either option. There is no reason to establish interactions among modules by using their concrete implementations instead of their interfaces.

• Should I be casting objects in order to use them?

I hope not. If so, it's because the interface is not well-designed, and a new one should be created to avoid this behavior. The use of the instanceOf function is also not desirable at all.

• Is the interaction between modules guided by IF statements?

There is no reason for this to be the case. Your modules should be connected in a way that can be taken care of by the use of an interface and the correct dependency injection to solve their concrete implementations.