How can I tell how often Google crawls my site?
Just check the webserver logs if the visitor's User-Agent request header matches/contains Googlebot. There are lot of webserver log analyzer tools, either free or payware. Most of them are also able to categorize bots. From them all I've had the best experience with Google Analytics.
Signup for Google Webmaster Central, aka Webmasters Tools, verify your site, and wait for the stats to appear.