When SharePoint gets "sick," knowing how to find the underliying problems can be tricky and time consuming.
The biggest hurdle to getting to a solution is gathering enough information about the problem. The best place to start is answering the cardinal questions: Who? What? When? Where? Why?
There may be a very specific treatment, or there may be many. Of the many, some treatments may be better than others, or just another way to get to the same result, some may just be camouflage.
Sometimes, the treatment surfaces another symptom, or the treatment itself needs some additional diagnosis and treatment...
In any case, you will need tools. Check back here for information about useful tools and use cases where they were put to use to solve a problem.