Spero che queste dritte ti possano essere utili.

con Jscript usa:

isNaN Method
Returns a Boolean value that indicates whether a value is the reserved value NaN (not a number).

isNaN(numValue)

The required numValue is the value to be tested against NAN.

con VBScript usa:

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.

ciao ciao
Gabriele