Oracle does not provide builtin function to check if a string is a number.
However, there is a function named to_number which can convert a string into a number. If the string is not a number, to_number would return, otherwise it would return the numerical value of the string.

Here is a function that makes use of to_number() to check if a string is a number.

CREATE OR REPLACE FUNCTION Is_Number
(num_In_str  IN VARCHAR2)
RETURN BOOLEAN
IS
num  NUMBER;
BEGIN
IF num_In_str IS NULL  THEN
RETURN NULL;
END IF;

num := To_Number(num_In_str);

RETURN True;
EXCEPTION
WHEN OTHERS THEN
RETURN False;
END Is_Number;