Enable USB Debugging on your device and from command line type adb devices. If everything is OK, the response should be:

List of devices attached
1234567890 device

Where 1234567890 is the device's id.
If multiple devices are connected, you should see all of them:

List of devices attached
1234567890 device
2222222222 device
...

When connecting a device for the first time, you'll get a pop-up window on your device, asking you to approve the connection.