Detect External Power on VIN

Is there a way for the Duemilanove to detect whether it's being powered by USB or through the VIN? I am using in an environment where my board is hooked up through USB but is controlling some servos, and only want it to operate the servos if an external power source through the VIN pin is turned on.

Thanks!!

I'm having a problem getting this to work right.. I'm using a mutlimeter and can't detect a change on IC5 PIN7 when I switch from USB to external power. I am not using the large power jack, I'm plugging into the "Vin" pin on the Power header - will that make a difference?

No, I want Vin to override the USB power. What voltage does Vin need to be to override USB? I'm supplying about 5.5V on Vin...

I can't tell what the orientation is of IC5.. is Pin 1 on the side closest to the Atmega or is IC5 facing away?

No, I want Vin to override the USB power. What voltage does Vin need to be to override USB? I'm supplying about 5.5V on Vin...

About 7volts or more.

ahhh........so that must be my problem... i need to supply a higher voltage. thanks guys