Any recommendations on how to best test our current workload of reads and writes? Also, if we are pretty certain we will need horizontal partitioning / sharding

Any recommendations on how to best test our current workload of reads and writes? Also, if we are pretty certain we will need horizontal partitioning / sharding, would it be better to wait for Neki before considering a move? For context we are on Aurora Postgres right now, with several read replicas.