Thats not hard , when there is new content more often on a website, google visit it more regularly, if there a website where there are tonnes of visitors but no new contents for month, google then visit it less frequently. However using xml sitemap you can tell google to how often check for new content .
Generally its better to create sitemap on your own site by coding it, wordpress has plugin for sitemap, smf has plugin / mod for sitemap and so do other softwares.