I have a simple question: How Google check the last modified date of any webpage in my website. If XML site map is the answer, then what happens to those sites which don't have xml site map ?
Look at any list of files on your computer. . . you'll see the file date. The column header is 'modified'. Alternatively, right click any file on your computer and select 'properties'. There's a created, modified, and last used date. The OS tracks all this. All googlebot needs to do is 'get fileInfo' which is a standard function in all programming languages.
When Google crawls your site, it compares your pages with the pages they crawled last time (the cache files).
But how they come to know when i have updated my content that what i am talking about. The last modified date is compared with last cached date of Google. If modified date is older than that of cached date then google passes the url for next crawl as they thing it's waste of time to crawl the same content as it was in previous crawl. So how Google come to know when i have modified my website content ?
Google constantly crawl ALL available content. Google doesn't say: "Oh, I have been at this page before, so I'm not gonna crawl it again".