Knowing existing documentation

Existing documents can be a great source of information, even though they can also have their issues. You should reserve some time to at least get familiar with all the existing documents related to your work. Chances are that there are some requirements hidden in them. On the other hand, keep in mind that the documentation is never perfect; highly likely it will lack some significant information. You should also be prepared for it to be outdated. There is never one source of truth when it comes to architecture, so aside from reading documents, you should have lots of discussions with the people involved. Nonetheless, reading documents can be a great way of preparing yourself for such discussions.