How to know whether a module requires RankNTypes or ExistentialTypes if it is not explicitly mentioned?

In the source of the memocombinators package, for example, no LANGUAGE pragma is explicitly mentioned and when trying to import it I get errors because 开发者_如何转开发the extension which supports the explicit forall statement is not included. How do I know which extension is required in this case ?

Check the memocombinators.cabal file: http://hackage.haskell.org/packages/archive/data-memocombinators/0.4.1/data-memocombinators.cabal