How to check value is a Number in PLSQL

Use TO_NUMBER function in PL/SQL.

Example:

DECLARE 
   value_     VARCHAR2(100);
   n_value_   NUMBER;
   
   CURSOR get_str_value IS
      SELECT SUBSTR('2BP0127', 3, 1)   value 
      FROM   DUAL;
BEGIN
   OPEN  get_str_value;
   FETCH get_str_value INTO value_;
   CLOSE get_str_value;
   
   BEGIN
      n_value_ := TO_NUMBER(value_);
      Dbms_Output.Put_Line('TRUE');
   EXCEPTION
      WHEN OTHERS THEN
         Dbms_Output.Put_Line('FALSE');
   END;
END;