Languages belonging to the same family obviously share many features, most of which were inherited from their common ancestor. But, considering that languages of the same family also are usually spoken in the same geographical region, some shared features are the result of horizontal spread (areal features).
So, how do linguists tell which is which? What are the clues that linguists take into account to tell whether a certain feature was inherited or spread through an area of linguistic diffusion?
Just to cite one example that comes to mind: Indo-European ablaut. It's generally accepted that ablaut was already present in PIE and that it reflected in its daughter languages (e.g. English sing/sang/sung). How do linguists know for sure that this was not an innovation in one of the early daghter languages that all others have borrowed?