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I'm using Windows 7. I connect to the internet using both my Ethernet port and Wifi.

How can I know which interface is Windows using to connect to the internet? What if there are even more interfaces, like an additional USB Wifi adapters? Is there a general rule for this?

There are two things you need to look at:

1. The routing table (and its default interface).


2. The preferred network settings in windows.

The routing tables are easy. Start a shell and (e.g. start run cmd.exe) and use the command route print. One of the entries will have network destination 0.0.0.0 (that means every destination not explicitly listed). The IP in the column Interface is the IP belonging to the card use to reach the default interface.

IPv4 Route Table

======================================================================

Active Routes:

Network Destination  Netmask     Gateway        Interface      Metric

      0.0.0.0        0.0.0.0    192.168.1.252   192.168.1.12     20

I know this is a bit brief, but there is a nice long answer at in this post on our sister site.

That part is rather generic and works that way for every computer, regardless if it is running windows, CPM, DOS, Linux, BSD, OS X, etc etc.

The second point is windows specific.

Windows defaults to the link with the highest speed.

If you have wireless up to 54Mbit and wire (usually 100mbit or 1000mbit) then windows will use the wired card if it is plugged in, and fall back to the wireless card if it is not plugged it. (This is not always a case of wired over wireless, you can have a 100mbit wired connection and a 300mbit wireless connection).

This might even change when you are waling around with the laptop, or when someone moves between your laptop and the WAP. (Water, and thus human are great absorbers of 2.4 GHz radiation.). Thus the actual used connection can change at any time.

If desired you can manually select which network should be use by going to the local area connection settings under Adapters and bindings.

Edit to answer some of the questions on comments which got to long to answer without using several 'comment posts':

Can you say that the interface with the lowest metric in the route table is the one that is used? – awe

Yes.

If there is more than one path then windows will look first at the most specific path and use that. If there are multiple paths to the same destination then it will use the path with the lowest metric value.

This value may be manually set to indicate cost (as in €/MB), or the number of routers to cross (more a technical cost), or speed.

This post on Serverfault describes how windows sets the default values for the metric.

Other interesting posts are:

1. Technet: The IP routing table


2. Technet: The route command


3. Here on [SU]: How does windows 7 decide which route to take if there are 2 connections
   

Update for windows 8.1 (with thanks to Joseph Quinsey):

For Windows 8.1, to find the manual selection, you need:

• Network and Sharing Center


• Change adapter settings


• Press Alt
  


• Advanced


• Advanced Settings


• Adapters and Bindings,

or

• right-click Start


• Network Connections


• Press Alt,


• Advanced


• Advanced Settings


• Adapters and Bindings,

On Windows, check in realtime using SysInternals TCPView. Find the application process in column Process. Then look at the column Local Address. That IP Address in Local Address is associated to a particular Network Interface.