How can I monitor network traffic in an all Mac home network?

You can try NetUse Traffic Monitor. It uses SNMP to get network usage data off your router and draws realtime and historic data.

I've been having the same problem with Google plus experiencing sporadic streaming hiccups on my Apple TV. After looking around and following a few links here, I ended up referencing traffic via Airport Utility » Base Station » Logs and Statistics. The same dialogs can also be accessed via the Advanced button along the top of the floating dialog box. Here you will find info on devices that have been connecting to your WiFi/Airport and last time each one connected.

That said, I didn't see any external access to my network so I'm still not sure what is causing my problems. I do plug into a network within my condo that connects thru Dish Network. The Google problem may be a result of traffic being generated by other tenants, but that still doesn't explain sluggish streaming within my LAN.