How can I ensure that ClickHouse distributes queries across all replicas?

distributed queries by default use only one live replica per shard
you can change it with SETTINGS max_parallel_replicas=3
and check
SELECT hostName(), count() FROM db.distributed_table GROUP BY ALL

Do I need to configure a specific load balancing strategy for read queries?

use <load_balacing> only if you understand why do you need it, for example nearest_hostname usually used for geo-distributed cluster to select from the same region which encoded in DNS hostnames in <remote_servers> <host>

Is there any way to confirm which replica is being used for queries?

SELECT hostName(), count() FROM db.distributed_table GROUP BY ALL