It is regularly a need to have more information about what is in the index. We even developped a scheduler job to look at the latest changed documents to verify if the index is up to date which we used for cluster mode (which could lead to some documents missing to be indexed).
Once we installed the lucene navigation tool to look at the index.
The most important is to know how many documents there are, to see if changing documents actually does make the number move. However it would be very valuable to be able to navigate the index to understand if the search is not finding a document because it's missing from the index or if the search is badly looking it up.