Consider:

 > Canary object testing is a technique whose purpose is to answer the questions "Is my data being replicated?" and "How long is it taking, roughly?" To accomplish this, you can use so-called "canary objects" — testing data which is not used or modified by your application, but which is stored in the same location —to monitor for the amount of time taken for changes to propagate across replication.

The key concepts to get across regarding canary objects (IMO) are that they live next to your real data, they aren't real data themselves, and watching how they progress through replication can give you an indicator of trouble, and a *rough* idea of how long realtime replication is taking for any given change at the time you make it.