Why it matters: You need to outline clearly what is content and what are controls — that is, what users can interact with. The indicators can be size, color, positioning, font, etc. Font is an important one: with different font styles or families, you'll help users not to get confused and easily identify which elements interact with.
How to test it: Identify all controls in your product/website and make sure they are standing out from the content. When running usability tests, check whether users are having difficulties interacting with controls. Check the example: