It would be quite easy to add debugging output to the digest sending machinery.
Do you have a staging site available, where one can test without compromising your production instance?
There, one could test if timeouts in queries occur, for example. Naively, I would expect timeouts to be seen in the logs.
One could further enhance the query that builds the targeted people with statistical output.
All this is not difficult in a sandboxed environment but can be dangerous in production.