Macbook is connected to wifi and tethering from iPhone. Which connection is it using? When on wifi with my Macbook, sometimes I plug my phone in via USB and both connections say they are working.  How do I know which connection my Macbook is using?  Further, how do I force my Macbook to prefer the wifi connection?



I'm running Mountain Lion 10.8.4 on a Macbook Air Mid 2011, and iOS 6.0 on an iPhone 5.
The preferred connection is shown at the top of your list.  This doesn't mean that the iPhone USB connection will never be used; but, it makes it less likely.

You can control which connection will get precedence with the gear icon at the bottom of the list window you showed, and select "Set Service Order".