Weight is meaningless (for testing). The UCI has a minimum weight that is well in excess of what modern carbon fiber layups allow. So, all pro bikes have extra weight built in, usually on the order of several pounds.

But, pulling the seatpost or crankset/bottom bracket should make it pretty obvious if a bike has a motor hidden inside the frame.

And it appears the UCI has started using some sort of x-ray device for quick tests in the paddock area.