Product data update check

There are several ways to check if new product data is available:

Separate parameter - You could use the download link parameter "&GetUpdateDate=yes". Please note that by adding this download link only the date of the last update will be generated.
Head - Using the download link in connection with the HTTP request method HEAD will provide you with a Header and a "Last-Modified" column.
If-Modified-Since - If you were to put values in the "If-Modified-Since" column when downloading in the HTTP-Header, you would then get the lastest new product data (if available). Please note that this is only a string comparison. If you were to decide for this option, please ensure that you save the value of the "Last-Modified" column from the header of your last successful download as a string.
XML-file - Via http://productdata.zanox.com/StatusReport.aspx?OutputFormat=xml you will get a list of specific update times for product data (in XML format).
zanox Web Services - enabled you to perform incremental updates for any product data feed. (SOAP / REST)

You can find more information on product data feeds and downloads including a PDF document here.