I can tell the minute DD has had preservatives/colours/flavours (even the so called "natural" ones). Even a dot of tomato sauce, or some soy sauce with caramel colouring in it, in her dinner. She gets aggressive, impulsive and hyperactive. She can't sleep and tells me it's because she feels "crazy".
I haven't been able to eliminate them all together (her dad and his family don't understand and let her eat whatever when she is with them) but minimising it as much as possible has made a huge difference.