You could use the onBlur and onFocus() event handler to get notification of when the text has changed. Then test the "form control name to get the actual form control and test to see which one it was. Then get the value from the control and do tests like isNaN(value) to see if it is a number and such.
Here is an example...
[This message has been edited by Heath Lilley (edited October 17, 2001).]