It's in the kiss
Sadly we won't ever see much kissing-on-the-lips between them as it goes against royal tradition and was reserved for the wedding day. Despite that they still manage to demonstrate synchrony in a peck on the cheek. If you're not in sync it's easily messed up - banging noses, kissing ears, etc. Kate guides him slightly with a hand on his shoulder - signalling she does her part in public to keep things smooth. But the most positive sign is her smiling eyes.