Checking if a field has been changed using Suitescript

Hi -

I'm working on a server-side script which needs to check if a field has been updated since the last time the user submitted the record.  It appears that the function nlapiGetOldRecord should do the trick.  However, that function seems to be returning null.  Any suggestions on this function would be very appreciated - if it makes a difference, I'm trying to call it after Submit.  Any other approaches to the same problem would also be welcome. 

Thanks!

--Tim


Here's my code:

// Get the Current Record
currentRecord = nlapiGetNewRecord();
// Get the Customer
customer = currentRecord.getFieldValue('entity');
// Get Ship Date
shipDate = currentRecord.getFieldValue('custbodyscheduledship');