There's most likely a more elegant way, but this does find every record with a letter in it, both upper or lower case:
select * from your_table
where UPPER(your_field) like '%A%'
or UPPER(your_field) like '%B%'
or UPPER(your_field) like '%C%'
or UPPER(your_field) like '%D%'
or UPPER(your_field) like '%E%'
or UPPER(your_field) like '%F%'
or UPPER(your_field) like '%G%'
or UPPER(your_field) like '%H%'
or UPPER(your_field) like '%I%'
or UPPER(your_field) like '%J%'
or UPPER(your_field) like '%K%'
or UPPER(your_field) like '%L%'
or UPPER(your_field) like '%M%'
or UPPER(your_field) like '%N%'
or UPPER(your_field) like '%O%'
or UPPER(your_field) like '%P%'
or UPPER(your_field) like '%Q%'
or UPPER(your_field) like '%R%'
or UPPER(your_field) like '%S%'
or UPPER(your_field) like '%T%'
or UPPER(your_field) like '%U%'
or UPPER(your_field) like '%V%'
or UPPER(your_field) like '%W%'
or UPPER(your_field) like '%X%'
or UPPER(your_field) like '%Y%'
or UPPER(your_field) like '%Z%'