determine if value in field is number

From: Matt Tapia (mtapia_at_inbizservices.com)
Date: 04/23/04 

Date: Fri, 23 Apr 2004 10:35:29 -0500

I have a varchar field in a table that could contain values such as 123,
345, ABC, or CDF

I want to do a select statement on the table to return records where this
field could be number (123, 345) and exclude those that are not (ABC, CDF).
Is there an IsNum function or something like that?