ActionScript 2.0 :: Detecting Var Output "NaN"?

I need to be able to detect if the input a user types into an Input text field is not a number (NaN). I thought this would be a breeze, but I am having trouble. First I created a function that sets a new variable equal to the input value multiplied by 1.

Code:
valuetest = value*1;

For testing purposes I placed a temp text box on the stage (var: valuetest). Sure enough, when the function is called and if value contains anything but numbers, valuetest becomes NaN. Then later on the timeline I check what valuetest is.

Code:
if (valuetest == "NaN") {
gotoAndPlay("error"); }

Does not do what it supposed to. I tried doing this several ways, including putting all the code into the function, but I can not get it to "see" the NaN as a variable. I replaced the "NaN" in the if statement with a number and it works when I input that number... so I suspect that the code is correct and the problem has to be something to do with how Flash deals with a NaN output.