Existence of Missing Index in MySQL. Does it exist?
There is another article here: https://stackoverflow.com/questions/30626876/how-to-find-missing-indexes-in-mysql

I, too, would recommend using the explain plan and looking at results.  I have had instances where adding an index to a table actually slowed the results down to a crawl.

Here is an interesting way to look up columns that could possibly need indexed:
http://www.youdidwhatwithtsql.com/spotting-missing-indexes-mariadb-mysql/2036/

Again, like the author said, "a query I use for MySQL / MariaDB to spot any columns that might need indexing" [emphasis mine]