Two methods. One involves using a collection of easily acquired electronics, see here. This approach is beyond my skill set, so I use a second approach that works fine for me, at least:
I use the Online Tone Generator but any sine sweep tool should do. With the Online Tone Generator set the volume to quiet conversation but not whisper quiet. Move the slider slowly and steadily left or right. Listen for any meaningful departures – either dips or rises. It’s that simple. However, it takes a bit of practice to learn to hear very gradual rises or falls in loudness. And also to distinguish what’s actual loudness vs the psychoacoustic softness of low frequencies and the psychoacoustic piercing-ness of high frequencies.
That said, there is a fundamental problem with your stated purpose. There literally is no such thing as a single neutral frequency response for headphones. People literally hear headphones differently from each other and the differences are not subtle. You may as well tell people that only size 46 extra-wide shoes are comfortable. I’m happy to explain the technical reasons why this is true for headphones yet not for live or loud speaker listening, but that gets into tl;dr territory very quickly.
As a reviewer you need to remind people that they literally hear differently than you do, especially at higher frequencies. I suspect this also carries over into sound stage and imaging perception to at least some degree.