How can I tell if a Google Map RSS feed has changed?

Much to my surprise, if I look at the response headers 开发者_StackOverflowfor the RSS representation of one of my Google Maps, there is no Last-Modified header. I want to do a conditional GET of the RSS (in C#).

I'm surprised because I thought RSS files would be especially likely to contain information in the response headers about their last updated time, given that they tend to represent resources that are expected to change over time.

So, how can I tell, without a full download of the RSS file, whether it has changed since the last time I got it? Or is that not possible?