Migrating Parts of Application Stack to Public Cloud

I am working on Data Center migration to AWS and I have a challenge to lay-out a strategy on how to slice the workloads. Hybrid is a Reality and a given in Enterprise IT, at least as a transitional state.

At a high-level I am going to split between tiers, front (cloud) and backend (on-premises). There is pros/cons as any decision, but I am particular interested on how can I identify chatty applications when communicating with the backend or specifically databases ‘cause they were built for env where the two sit very close to each other. One front-end request can issue dozens of requests to the backend, so splitting such a chatty channel is likely to result in poor performance.