Transactional replication provides the tracer token feature, which provides a convenient way to measure latency in transactional replication topologies and to validate the connections between the Publisher, Distributor and Subscribers. A token (a small amount of data) is written to the transaction log of the publication database, marked as though it were a typical replicated transaction, and sent through the system, allowing a calculation of: 

•  How much time elapses between a transaction being committed at the Publisher and the corresponding command being inserted in the distribution database at the Distributor.
•  How much time elapses between a command being inserted in the distribution database and the corresponding transaction being committed at a Subscriber.