uamv

Common Links allows you to add regularly used links to the search results when linking text content of your post.
Add your common links from the Tools menu. Then, when linking content, begin typing the link title or URL to pull up and insert the link.
Note: Entering <code>.</code> when querying links will not return results. Use spaces instead to match URLs.

Common Links

Adds custom links to the editor's link query

cubecolour

A simple and lightweight plugin to indicate external http and https links in posts, pages and custom content by appending a small diagonal arrow icon to the link.
No configuration is required just install and activate.

Indicate External Links

Very reliable has survived many Wp updates and just keeps working.

This plugin just keeps on working

Indicates external links in post, page and custom content.

Common Links vs Indicate External Links