Yoast SEO plugin probably is the most popular SEO plugin for WordPress. One of best features this plugin provides is content analysis for SEO. Unfortunately, this plugin analyzes only post content while our pages may have additional content in custom fields. This content is also outputted in the frontend and is visible to search engines and should be analyzed for SEO, too. Now, this can be done with Meta Box Yoast SEO.
Meta Box Yoast SEO adds a custom attribute to the custom fields when you register them with the Meta Box plugin to make them analyzed by Yoast SEO plugin.
Assuming you have an editor field (WYSIWYG) where you write a custom content A, a textarea field where you write another content B. A and B can include keywords for SEO. Yoast SEO will analyze post content + A + B and give you higher SEO score and eliminate problems like the minimum number of words below 300, keyword density is too low, etc.
Here is a screenshot how the plugin works:
The idea of the extension comes from a support question by Piet Bos on Github. Thank Piet!
The extension is free and available on WordPress.org, you can download it here.
How To Analyze Content in Custom Fields for SEO - Using Yoast SEO and Meta Box 
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