I think this might work. Written on the fly.
(defun IsStrNums (String)
(and
(mapcar
'(lambda (x) (>= 48 x 57))
(vl-string->list String)
)
)
)
I think that is the right function to use (vl-string->list), but not sure.
Edit: Change the test to greater than or equal, not less than or equal.