Is the code extensible?

Rarely you're going to find a module that does exactly what you need out of the box.

Usually there will be some configuration options, or it will get you to 80% or 90% and you need to fill in any gaps.

If you need to extend, overwrite or customise a module, have its Developers made it possible or easy to do that?

Good code should be easy to change or extend.

Are there hooks you can implement or events you can subscribe to and run your own code?

Are there Interfaces and base classes you can implement or extend that allow you to use design patterns such as the Decorator pattern?

Are there public services you can use instead of re-writing and duplicating existing functionality?

Is there documentation or examples showing how to extend the module?

Is it a stable foundation for you to build on?