There are lots of ways to do this:
1. In your project you can add a reference to Microsoft.VisualBasic and use the IsNumeric function that is contained within that Namespace.
2. You can loop through the string char by char and call Char.IsDigit()
3. You can use regular expressions to determine if the string contains only numeric values.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^
Technical Editor for: Professional Search Engine Optimization with ASP.NET