Presa da support.microsoft.com :
codice:
    Private Shared Function IsNumeric(ByVal Expression As Object) As Boolean
        Dim isNum As Boolean
        Dim retNum As Double
        isNum = Double.TryParse(Convert.ToString(Expression), System.Globalization.NumberStyles.Any, System.Globalization.NumberFormatInfo.InvariantInfo, retNum)
        Return isNum
    End Function 'Controlla se un valore è numerico
Vedi link per approfondimenti