Download your server access logs = ie /home/username/logs/example.com/http.n/access.log
These will show you the hits to your website, and obviously if you are getting a lot of robots trying to post comment spam or exploit vulnerabilities it will stick out like a sore thumb. They don't waste time downloading images or CSS files and most likely POST out of the blue or go straight for the login in page.
Obvious things to do when setting up a blog: don't let it automatically perform an "update" service that lets various websites know yours exist. And use Apache directives to block posting comments and trackbacks instead of relying on akismet and moderation. If the hit doesn't result in an immediate response from Apache, you're spending CPU time running PHP code and up goes your CPU minutes.
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