I'm assuming that Behemoth and Diamondback have the same trains. On Behemoth, if you sit in the front left of any car, you can see a little section cut out with some lights in it. There should be 5 lights, the 4 to the left are for each restraint. So if you're sitting in the front left, the first light should illuminate when the restraint is down enough. If it's not, it won't light up. Someone maybe check this out and see if there's any truth to this with Diamondback.
EDIT: After some Behemoth riding today (12 rides), I snapped a picture to help explain what I mean. http://img14.imageshack.us/img14/5219/3492...704aba6667o.jpg