Hello! Could you please help me to clear up this: how to show Google when a webpage was last modified, so it will appear in searching results? Is sitemap or Last-modified useful for this? Could you recommend other options? Thanks in advance!
I'm not sure I understand.... Google doesn't show "last modified" timestamp does it? The following prefix will show when Google last crawled the page: http://webcache.googleusercontent.com/search?q=cache:**URL_HERE** Code (markup): (Replace **URL_HERE**, of course)
Google's Matt Cutts gave a great tip on a recent video for how to get a newly-revised page crawled quickly (sorry, I don't have the link). Go to Google Webmaster Tools > Health > Fetch As Google. In the "Fetch" box, finish adding the complete URL of the page you have recently revised. Click "Fetch" and wait for a few seconds. Soon a "Submit" button will appear. Click that, and you have just asked Google to crawl that page! It's the fastest way I know to get a newly revised page crawled.
A small addition on your post Jim: a backlink will also be a notice for the spider to crawl that certain webpage. So when you earn a backlink on an (active) authority website you will be also crawled fast (maybe even immediately depending on the source).
If you add any new content in your website or blog then use Google Web Master Tool option Fetch as Google to quick crawling and indexing. In this way Google see your last added content.