On the Pure Core Mattress with Hybrid Cover, you can visibly see a difference between the two sides if you are looking at the core (under the hybrid cover and the mattress inner cotton cover). The firmer infant side has a thinner top layer, while the toddler side is softer and thicker.
On all our Pure Core full-size mattresses, if you squeeze the two layers, the infant side layer is firmer to touch than the softer toddler side layer.
Mini Crib mattress and all Coco Core mattresses are not dual sided.