WWW/Server Problems

Ok so I am trying to limit downloads to only when its absolutely necessary. I know the exact URL where the file is located, how can I get the timestamp of when the file was last modified WITHOUT having to download it? (which would only change the timestamp anyway) I'm using amazon web services which only allows me to make requests in async. Problem with that is I want to compare the timestamp to the timestamp on my device, if I wait for async the compare code will have run before the info is returned to compare against it.

Is there anyway to reference a file without using www?

Does www actually download the file or is there a way to use it to get the date time first?