You could check by making sure each character is between 0 and 9, + or - and, if accepting decimal points '.'
Letters would be harder depending on how international you want the check.
It may be easier to just interface with the is isnum and isalpha C commands using INLINE
INLINE
extern "C" isnum(char one);
extern "C" isalpha(char one);
ENDINLINE
and then write a function to check each letter for numbers and/or characters