How To Check A Field For A Value Of "undefined"?

How To Check A Field For A Value Of "undefined"?

I have a simple webpage with a form on it. The form has two select boxes for use as filters, when submitting a query to the server. Each select has an "onChange" handler coded for it dynamically, when the form originally loads (using document.ready). Whenever either selects' value changes a request is submitted using both selects current values. My problem seems to be that whenever I click a value in one select, without the other select already having had one of its values selected, the select which has not previously had a value chosen for it, is having a value of "undefined" returned from it and used as a parameter in the query that it is submitted with and so nothing is returned from the query using that bad value, as a parameter. I've already tried checking the field value for an empty string(value="" or length not > 0), but that didn't work.
 
So my question is how can I check both fields for a bad or invalid value of "undefined" so I can react appropriately and either display and error or format a default value for that parameter in the query?
 
Thanks for the help.