Be sure to have a Google Webmaster Tool account and add your site for tracking. The best way to view if there’s any error in the index is to view the “index status” of GWT. Look at the total index, if your site has continuous content generated then you should see an uptrend. If you start seeing downward trend, its time you look at what’s not being indexed.
Viewing the crawl errors page will show you the problem pages. Go over to the “crawl stats” highlights the activities of the bots. If you had some problems with duplicate pages or removed pages. There may be an increase in the “pages crawled/day” and “Kilobytes/day” by the bots to ascertain. “Time spent download/page” is important for the page load speed, the less time the better for user experiences.