Android: how to know file located in web server is modified?

I am parsing Xml file located in web server and storing parsed data in to database. for my app I am using data from database. I need to parse the xml file only if the file is modified otherwise no need to parse. So how can I know the file is modified? I know I can use "if-modified-since" header. But I need some examples of "if-modified-since" header
please help me.......