The answer is running Google DNS reverse lookup.
Let's see some straight forward examples:
Example 1:
> host 66.249.66.1 1.66.249.66.in-addr.arpa domain name pointer crawl-66-249-66-1.googlebot.com. > host crawl-66-249-66-1.googlebot.com crawl-66-249-66-1.googlebot.com has address 66.249.66.1
Example 2:
> host 66.249.90.77 77.90.249.66.in-addr.arpa domain name pointer rate-limited-proxy-66-249-90-77.google.com. > host rate-limited-proxy-66-249-90-77.google.com rate-limited-proxy-66-249-90-77.google.com has address 66.249.90.77
Reference: //support.google.com/webmasters/answer/80553?hl=en