Will this help you?
without UDF or RegEx there's no easy way to determine that.
You could create a monster CASE to check each character for it's character class:
CASE WHEN SUBSTRING(col FROM 1 FOR 1) NOT BETWEEN '0' AND '9'
OR SUBSTRING(col FROM 2 FOR 1) NOT BETWEEN '0' AND '9'
OR SUBSTRING(col FROM 3 FOR 1) NOT BETWEEN '0' AND '9'