As long as you content provider supports IContent, it can be indexed by Find (you can technically use find with non-content objects, but that's trickier). Once it is indexed, it can be searched. 

Thanks Quan. So if I understand correctly, Find will be able to index IContent data from custom content providers even if the data is not stored in the CMS database?

Also, do you know when data in the external data source is updated, would we need to manually reindex or would Find handle this automatically?

If you simply extend ContentProvider then your basic events (create, publish, delete etc.) should be covered by Find.