Читаем хелп
IsNumeric Function
Returns a Boolean value indicating whether an expression can be evaluated as a number.
IsNumeric(expression)
The expression argument can be any expression.
Remarks
IsNumeric returns True if the entire expression is recognized as a number; otherwise, it returns False. IsNumeric returns False if expression is a date expression.
The following example uses the IsNumeric function to determine whether a variable can be evaluated as a number:
Dim MyVar, MyCheck
MyVar = 53 ' Assign a value.
MyCheck = IsNumeric(MyVar) ' Returns True.
MyVar = "459.95" ' Assign a value.
MyCheck = IsNumeric(MyVar) ' Returns True.
MyVar = "45 Help" ' Assign a value.
MyCheck = IsNumeric(MyVar) ' Returns False.
и что из вышесказанного следует? То что IsNumeric вернет то именно что надо.