The Monitoring Console can tell you that. It's under Indexing->License Usage->Historic License Usage. Select "Index" from the "Split by" dropdown.
Hi
MC is the best option to start that analysing, but as is has limited to some amount of indexes etc. and you probably want to see more you can use it easily as a starting point! Just select e.g what @richgalloway said and then click that small magnifying glass on bottom of panel and it will open you a search window where you could modify that search as you need. You could e.g. show more individual indexes or extend historic time etc. with your "own" search.
r. Ismo