How can I monitor network in OS X? I met a problem recently.

Sometimes the Google shows me a page says that my computer is sending frequent requests to Google so that I must enter the number in the picture to continue to use Google's services.

Obviously, I would not do such thing.

The applications installed in my Mac are all from trusted sources(e.g. official websites).

I have no idea 'who' is doing such thing.

How can I monitor the network to find out the 'worngdoer'.
Try typing netstat in terminal.app
First I would type man netstat to get a better understanding of how it works and what it is capable of and what the results mean.

Try searching a tutorial about the command on stackexchange or youtube.