Verifying Googlebot

To verify Googlebot as the caller:

1. Run a reverse DNS lookup on the accessing IP address from your logs, using the host command.
2. Verify that the domain name is in either googlebot.com or google.com.
3. Run a forward DNS lookup on the domain name retrieved in step 1 using the host command on the retrieved domain name. Verify that it is the same as the original accessing IP address from your logs.

https://support.google.com/webmasters/answer/80553