ISNUMERIC will not do – it tells you that the string can be converted to any of the numeric types, which is almost always a pointless piece of information to know. For example, all of the following are numeric, according to ISNUMERIC:
£, $, 0d0
If you want to check for digits and only digits, a negative LIKE expression is what you want:
not Value like '%[^0-9]%'