Method 1:

Select length (translate (trim (column_name),' +-.0123456789',' ')) from dual ;

Will give you a zero if it is a number or greater than zero if not numeric (actually gives the count of non numeric characters)

Method 2:

select instr(translate('wwww',

'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',

'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'),'X')

FROM dual;

It returns 0 if it is a number, 1 if it is not.