DigitQ
DigitQ[string] yields True if all the characters in the string are digits in the range 0 through 9, and yields False otherwise.
See The Mathematica Book: Section 2.7.4. See also: LetterQ, Number.
Further Examples
DigitQ[string] yields True if all the characters in the string are digits in the range 0 through 9, and yields False otherwise. 
