there is the INT() function.
if ISNULL(INT([string field])) THEN (you have something other than numeric chars in string_field) END
INT() will return a numeric value if it can evaluate the string. Otherwise it will return NULL. So if ISNULL() evaluates to true, then you had something more than numeric characters in string field].
And probably, if you are looking to operate only on numeric strings, you would want to do:
IF NOT ISNULL(INT([string field])) ...
Can we attempt the following calculated field?
Firstly it converts your field to an integer. Anything that's all numbers can do this. Anything with any strings cannot.
Then we check to see if it is null, we return either true or false for this part. True are those that couldn't be converted.
Finally we use the IIF function which checks to see the first expression returned true or false and then assigns 'no' or 'yes' depending on our result.