How to check for uppercase letters in MySQL?

REGEXP is not case sensitive, except when used with binary strings.

http://dev.mysql.com/doc/refman/5.7/en/regexp.html

So with that in mind, just do something like this:

SELECT * FROM `users` WHERE `email` REGEXP BINARY '[A-Z]';

Using the above example, you’d get a list of emails that contain one or more uppercase letters.