The goal of assessment is decision making. Managers decide the strategy. Architects decide the technical direction. Developers decide the implementation. Daily.
Effective decision making requires accurate information. But, software systems are large. Thus, manual inspection does not scale. You need analysis tools.
Software systems are complicated in specific ways. The system details are essential for your decisions. You need custom tools that provide the information that matters. Craft them.
Analysis tools are important to handle the scale of problems, but in the end, assessment is a human activity. Regardless of how smart the tools are, it is still the human that has to infer the relevant information and transform it into action.
Not all problems are created equal. Some are of continuous concern. They deserve daily attention. Some only appear once. These require different approaches. Learn their particularities. Deal with them individually.
Decisions are made everyday. It's just that the effort is implicit and unoptimised. Assessment must become explicit and made integral part of the overall process. You need to approach it systematically. You need a method.