Copy-paste your website or landing page copy into Copytesting.
People from the selected audience will mark parts of your copy as good (interesting, emotional, compelling, etc) or bad (boring, bland, unclear, etc).
Copytesting will highlight parts that are statistically significantly good or bad, or mediocre. This tells you which parts of the copy to focus on.
For every rating copy testers will add comments for what exactly is unclear about something, or what makes itboring, or what's the problem with a particular sentence.