If each store instance could declare itself as being local, either by detecting or via config, then the cache api could make some suggestions:

1) If a cache definition is marked as canuselocalstore, and there happens to be instances of stores which are local but it isn't using them then it could suggest to use them

2) If canuselocalstore is false and it is mapped to a local store then it could warn

Some cache stores like APCu are always local and could be hard coded as such. Other stores like redis or files would need to be manually told in their configuration if they are local or shared. Ideally this toggle is part of the base store and APCu would override it.