Currently, the Reindex Search option in App manager (or Reindex All) gives no indication of progress of indexing, and no indication of indexing having finished. It would be much better if the UI gave an indication of progress - number of records/total. Or displayed the date/time when indexing complete.
For Solr indexing, a progress indicator would need to display feedback from Solr on progress, rather than just when Liferay sends records to Solr.
Additionally, indexing a record in can fail. Several reasons for this. For the document library, there are lots of reasons why Liferay can't extract text from a file (everything from PDF errors to Word documents with passwords). And the indexing in Solr can also fail - for example when processing is processor heavy, IOErrors happen frequently). There is currently no validation that a record has been successfully indexed.
It would be much better to have an indication that a record has been successfully indexed. Probably the easiest way of achieving this would be to query Solr for all the records of a type and cross reference against the records in Liferay.