ASCII is the most fundamental character set that has been around since the early days of command line interfaces. There are 128 (0 – 127) most basic ASCII characters such as a-z, A-Z, 0-9, and all the printable punctuations you can type out by a single strike of your keyboard.
As all ASCII characters have an internal decimal value of 0 – 127, which is 0x00 – 0x0F in heximal values, you can find all the non-ASCII characters in my_column by query:
SELECT * FROM my_table WHERE NOT HEX(my_column) REGEXP '^([0-7][0-9A-F])*$';
On the other hand, if you wish to find all records that a certain column (my_column) contains ASCII characters:
SELECT * FROM my_table WHERE HEX(my_column) REGEXP '^([0-7][0-9A-F])*$';