Confirm that someone entered a number (and only a number) into a field?

I have an iPad application with input fields. Some of the require a number be input, such as 6 or 10.25. I then do math with the inputs when the user clicks a button, by casting the value to Number().

How can I check that the user entered a valid number (as opposed to a word or something like 1'6"?

SDK 2.01, OS 10.7.3