thewatermethod

Displays a list of images in the media library without alt text, and an admin message reminding you to add it.
<a href="https://github.com/thewatermethod/wcw-alt-tag-audit" rel="nofollow ugc">View the code on Github</a>

Alt Attribute Audit

Displays a list of images in the media library without alt text, and an admin message reminding you to add it.

Kamer DINC

Compatible with the most popular plugins. Please report if you faced any problem with any plugin. 
Inserts automatically alt tag/attributes to HTML if alt attributes are empty or somehow missing. 
Works only in single page.
Note: This plugin is written only in PHP. So you can except a better SEO visibility and better crawl statistics.
Credits

WP Auto Alt Image Tag

(PHP Way) This plugin insert to html missing image alt tags/attributes based on each attachment title or filename.

Alt Attribute Audit vs WP Auto Alt Image Tag