How to tell which Plugin is displaying?

It can be less than trivial to match output to a source code responsible in WordPress.

The typical cases are following:

  1. The output is produced by template file directly. The locating template responsible is often about just searching for full or partial text match. From the other side template files can be identified among all files being loaded.

  2. Output is produced by function call in template file. This is not far from previous case, but can be slightly trickier since code definitions is now in separate place from code execution.

  3. Output is produced by function call, hooked to an action/filter. This can be nightmarish to unwrap, especially in cases of things getting nested.

In a nutshell good search tool is always a solid start. After that it’s matter of what you typically work with and what tools are fitting for that. Personally I find Query Monitor plugin striking good balance in providing debug information but not too much information.